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BY 

J.  A.  FLEMING,  M.A.,  D.Sc,  F.R.S 
U 

PROFESSOR    OF   ELECTRICAL    ENGINEERING    IN    UNIVERSITY   COLLEGE, 
LONDON, 

MEMBER    OF   THE    INSTITUTION    OF    ELECTRICAL    ENGINEERS. 


The  Illustrations,  Designs,  and  Text  of  these  Lectures  are  entered  at  Stationers 
Hall,  and  are,  therefore,  copyright. 


THE    ELECTRICIAN"    PRINTING   &    PUULISIUNCi    COM1ANY, 

LIMITED, 

SALISBURY    COURT,    FLEET   STREET. 
1899. 


WORKS  BY  DR.  J.  A.  FLEMING. 


Alternate-Current  Transformer,  Vol.  I.  New  Edition. 
Alternate-Current  Transformer,  Vol.  II. 

Magnets  and  Electric  Currents. 

An  Elementary  Treatise  for  Electrical  Artisans  and 
Science  Teachers. 

Electrical  Laboratory  Notes  and  Forms. 

Consisting  of  20  Elementary  and  20  Advanced  Papers 
for  the  use  of  Students  in  Electrical  Engineering 
Classes.  Intended  as  a  help  to  the  Teacher  and  his 
Assistants.  A  guide  to  the  Student. 

Centenary  of  the  Electric  Current,  1799-1899. 


PREFACE 


TO  THE 


SECOND     EDITION. 


HE  following  pages  are  a  tran- 
script, with  considerable  addi- 
tions, of  a  course  of  Four 
Lectures  on  "  Electric  Illu- 
mination, "  delivered  by  the 
Author  in  1894  to  afternoon 
audiences  at  the  Royal  Insti- 
tution, London.  They  are  in 
no  sense  presented  as  a  com- 
plete treatment  of  a  subject 
which  has  long  since  outgrown 
the  limits  of  a  single  small  book 

or  course  of  lectures.  The  original  aim  was  merely 
to  offer  to  a  general  audience  such  non-technical 
explanations  of  the  physical  effects  and  problems 
concerned  in  the  modern  applications  of  electricity  for 
illuminating  purposes  as  might  serve  to  further  an 
intelligent  interest  in  the  subject,  and  perhaps  pave 
the  way  for  a  more  serious  study  of  it.  In  preparing 
a  new  issue  of  the  book  the  Author  has  added  to  each 


_85299 


ii.  PEE  FACE. 

chapter  paragraphs  which  serve  to  bring  up  the  infor- 
mation more  into  line  with  recent  practice,  and,  with- 
out departing  from  the  elementary  character  of  the 
work,  made  the  necessary  corrections  to  preserve  the 
text  from  being  antiquated  as  to  statement. 

Many  of  those  who  were  present  at  the  Lectures 
expressed  a  desire  to  be  again  able  to  refer  to  the 
descriptions  and  illustrations  then  given,  and  at  their 
request,  and  in  the  hope  that  they  may  be  of  use  to 
others,  the  lecture  notes  have  been  revised  and  pub- 
lished. The  aim  throughout  has  been  rather  to  deal 
with  principles  than  with  details,  and  to  give  such 
general  and  guiding  explanations  of  terms  and  processes 
as  are  essential  for  a  grasp  of  the  outlines  of  the  sub- 
ject. For  this  purpose  simple  drawings  have  been 
given  of  as  many  of  the  experiments  and  apparatus 
used  as  possible.  For  permission  to  reproduce  a  set 
of  photographs  of  views  of  the  interior  of  rooms 
artistically  illuminated  with  incandescent  lamps,  and 
which  are  to  be  found  in  the  second  Lecture,  the  author 
is  indebted  to  Mr.  A.  F.  Davies,  under  whose  guidance 
these  particular  instances  of  electric  lighting  were 

carried  out. 

J.  A.  F. 

University  College,  London. 
November,  1899. 


CONTENTS. 


(For   Index  to    Contents,   see  pa  ye   267.) 


LECTURE  I. 

ELECTEIC  MEASUREMENTS    ... 

14  Illustrations. 


PAGES 

1—50 


LECTURE  II. 
ELECTRIC  GLOW  LAMPS       ...          ...          ...          ...       51 — 137 

40  Illustrations. 


LECTURE  III. 


ELECTRIC  ARC  LAMPS 


139—191 


14  Illustrations. 


LECTURE  IV. 
ELECTRIC  DISTRIBUTION       193 263 

39  Illustrations. 


ERRATA. 


The  reader  is  requested  to  make  the  following  corrections  : — 

Page  118,  last  line— 

for  "  Thompson  " 
read  "  Thomson." 

Page  133,  seventh  line  from  bottom — 

for  "  slight  resistance  " 
read  "  slight  increase  in  the  resistance." 

Page  135,  in  the  table— 

for  "26  and  25" 
read  "  2-6  and  2-5." 

Page  142,  seventh  line  from  bottom — 

for  "  Thompson  " 
read  "  Thomson." 


I  UNIVERSITY 


LECTURE    I. 


ELECTRIC  LIGHTING  in  Great  Britain. — A  Glance  Backward  over  Twenty 
Years. — Present  Condition  of  Electric  Lighting — An  Electric  Current. 
—Its  Chief  Properties.— Heating  Power.— Electrical  Resistance.— 
Names  of  Electrical  Units.— Chemical  Power  of  an  Electric  Current. — 
Hydraulic  Analogies. — Electric  Pressure. — Fall  of  Electric  Pressure 
down  a  Conductor.  — Ohm's  Law. — Joule's  Law. — Units  of ,  Work  and 
Power. — The  Watt  as  a  Unit  of  Power. — Incandescence  of  a  Platinum 
Wire. — Spectroscopic  Examination  of  a  Heated  Wire. — Visible  and 
Invisible  Radiation.— Luminous  Efficiency. — Radiation  from  Bodies  at 
Various  Temperatures. — Efficiency  of  Various  Sources  of  Light. — The 
Glow  Lamp  and  Arc  Lamp  as  Illuminants.— Colours  and  Wave- 
Lengths  of  Rays  of  Light. — Similar  and  Dissimilar  Sources  of  Light. 
— Colour-distinguishing  Power. — Causes  of  Colour. — Comparison  of 
Brightness  and  Colour. — Principles  of  Photometry. — Limitations  due 
to  the  Eye. — Luminosity  and  Candle-power. —  Standards  of  Light. — 
Standards  of  Illumination. — The  Cand)e-foot.— Comparison  of  Sun- 
light and  Moonlight. — Comparison  of  Lights.— Ritchie's  Wedge. — 
Rumford  and  Bunsen  Photometers. — Comparison  of  Lights  of  Different 
Colours.—  Spectro-photometers. — Results  of  Investigations. 


^HOEVER  undertakes  to  describe  the  re- 
markable progress  during  the  last  twenty 
years  of  the  art  of  electric  illumination 
must  certainly  direct  attention  to  three 
important  dates  which,  in  England  at  any 
rate,  formed  turning  points  in  the  history 
of  this  application  of  scientific  knowledge. 
^  In  the  year  1879  the  Government  of  our 

country  had  its  attention  directed  for  the  first  time  to  electric 
lighting  as  a  possible  subject  of  legislation,  and  referred  the 
whole  matter  to  a  Select  Committee  of  the  House  of  Commons, 
of  which  Lord  Play  fair  was  appointed  Chairman.  This  Com- 
mittee met,  and  took  voluminous  evidence  from  numerous  and 


2          ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

various  experts,  but  the  broad  conclusion  reached  in  the  report 
which  it  finally  presented  was  generally  the  expression  of 
opinion  that  there  was  no  reasonable  scientific  ground  at  that 
date  for  supposing  that  domestic  electric  lighting  had  obtained 
a  sufficient  footing  to  entitle  it  to  be  described  as  a  practical 
success,  and  that,  therefore,  there  seemed  no  useful  result  to  be 
attained  by  interfering  at  once,  by  special  legislation,  with 
electric  lighting.  Passing  over  a  gap  of  three  years,  we  find 
in  the  year  1882  the  whole  aspect  of  affairs  entirely  changed 
by  the  completed  invention  of  the  electric  glow  lamp.  At 
the  beginning  of  that  year  the  first  Crystal  Palace  Electrical 
Exhibition  enabled  the  public  to  properly  appreciate  the 
extent  to  which  the  then  perfected  electric  incandescent  lamp 
had  revolutionised  artificial  lighting,  and  also  the  charm 
and  beauty  of  that  illuminant.  Countless  details,  however, 
remained  to  be  perfected,  both  in  the  glow  lamp  and  in  the 
devices  for  generating,  distributing,  using  and  measuring  the 
electric  current  required  for  it.  From  the  commercial  point 
of  view  much  information  had  to  be  slowly  accumulated 
before  even  approximately  correct  opinions  could  be  formed 
as  to  the  revenue  to  be  gained  from  the  sale  of  electric  energy 
for  domestic  purposes,  and  there  was  at  that  date  but  little 
information  available  for  enabling  an  accurate  forecast  to  be 
made  concerning  the  probable  average  annual  consumption  of 
electrical  energy  by  incandescent  lamps  when  used  instead  of 
gas  jets  in  different  classes  of  buildings  for  illuminating 
purposes.  There  is  no  doubt,  however,  that  the  Electric 
Lighting  Act  of  1882,  though  much  abused  at  the  time, 
performed  the  important  function  of  preventing  the  survival 
of  immature  schemes. 

Between  1882  and  1888  exceedingly  important  improvements 
(some  of  which  it  will  be  necessary  later  on  to  examine)  were 
made,  and  in  1888  the  time  seemed  ripe  for  a  fresh  forward 
movement.  This  was  effected  by  the  pressure  brought  upon 


ELEGTRIG  MEASUREMENT.  3 

the  Legislature  to  repeal  one  of  the  clauses  in  the  Act  of  1882, 
by  which  revision  much  more  favourable  conditions  were 
created  for  inviting  the  support  of  capital ;  and  as  soon  as  the 
Electric  Lighting  Amendment  Act  of  1888  was  an  accomplished 
fact,  a  very  important  inquiry  was  held  by  the  Board  of  Trade 
in  May,  1889,  in  the  Westminster  Town  Hall,  London,  under 
the  chairmanship  of  Major  Marindin.  At  this  inquiry,  which 
lasted  for  eighteen  days,  the  whole  subject  of  electric 
lighting  by  public  supply,  especially  with  reference  to  the 
needs  of  London,  was  carefully  debated  by  many  of  the 
leading  scientific  and  legal  experts,  and,  as  a  result,  the 
Metropolis  was  divided  up  into  certain  areas  of  electric 
supply,  and  conditions  were  laid  down  under  which  the 
distribution  of  current  might  be  undertaken  either  by  Public 
Companies  or  by  the  Local  Authorities.  Up  to  the  date  of 
that  inquiry  the  total  amount  of  electric  lighting  in  London 
or  in  the  Provinces  had  been,  comparatively  speaking,  very 
small.  From  and  after  that  date  it  has  advanced  by  leaps 
and  bounds.  The  progress  made  in  the  use  of  the  incandescent 
lamp  as  a  means  of  artificial  illumination  is  shown  by  the 
following  figures,  and  graphically  indicated  in  Fig.  1. 

At  the  end  of  1890  there  were  probably  in  use  about 
200,000  incandescent  lamps  in  London,  but  at  the  end 
of  1898  rather  over  two-and-a-quarter  million  8  c.p.  in- 
candescent lamps  were  in  employment.  In  the  Provinces 
in  1890  there  was  hardly  any  electric  lighting  worth  men- 
tioning, whereas  at  the  end  of  1898  the  total  number  of 
lamps  in  use  in  connection  with  public  electric  supply 
stations  was  just  under  three  millions,  the  grand  total  for 
London  and  the  Provinces  being  5,206,000  8  c.p.  lamps 
or  their  equivalent.  There  are  at  the  present  time  (July, 
1899)  thirty-six  electric  lighting  undertakings  in  and 
around  London,  including  London  proper  and  Greater 
London,  These  are  in  the  hands  of  either  public  Limited 


4      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


Liability  Companies  or  of  the  Local  Authorities.  In  the 
Provinces  there  are,  at  the  same  date,  completed,  or  being 
completed,  150  electric  lighting  stations  in  as  many  towns 
and  cities.  In  these  the  electric  supply  is  likewise  given 
either  by  the  Local  Authority  or  by  a  Limited  Company. 
Whereas,  however,  in  London  the  majority  of  the  lamps  in 
use  are  supplied  by  Companies,  in  the  Provinces  it  is  the 
reverse.  The  Local  Authorities,  in  the  provincial  towns  to  a 


3,000,000    8  c.p.  Lamps 

I 

a.000,000  8  c.p.  Lamps 

1 

1,000,000   8  c.p.  Lamps 

1 

1 

I 

| 

1830        1891 


1892 


1893 


1894 


1895 


1696 


1S97 


18J8 


Fia.  1. — Diagram  showing  the  Progress  of  Electric  Lighting  in  London 
and  the  Provinces  in  Nine  Years.  The  altitude  of  the  vertical  black  lines 
represents  to  scale  the  number,  from  year  to  year,  of  8  c.p.  lamps  installed. 
The  firm  lines  are  lamps  in  London  and  suburbs,  and  the  dotted  lines  are 
lamps  in  the  Provinces. 

considerable  extent,  have  assumed  the  position  of  "  under- 
takers," as  they  are  called  in  the  Acts,  but  in  a  few  cases 
have  allowed  the  Provisional  Order  to  be  possessed  by  Public 
Companies.  Hence  in  the  Provinces  of  the  United  Kingdom 


ELECTRIC  MEASUREMENT.  6 

electric  lighting  has  largely  become  municipalised,  and  in 
many  cities  (such  as  Liverpool,  Sheffield  and  others)  where 
the  public  electric  lighting  has  been  originated  by  a  Limited 
Company,  the  Corporation  has  ultimately  purchased  the 
undertaking,  often  for  much  more  than  its  original  cost. 

In  the  ten  years  between  1888  and  1898  it  may  be  said 
that  the  business  side  of  public  electric  supply  and  the  art  of 
management  and  control  of  public  electric  generating  stations 
became  a  specialised  industry.  The  causes  contributing  to 
financial  success  or  failure  in  this  new  industry  were  carefully 
studied,  and,  side  by  side  with  this  attention  to  the  com- 
mercial questions  involved,  steady  progress  in  the  mechanical 
and  electrical  improvements  of  the  machinery  of  generation 
and  distribution  took  place.  Hence,  at  the  close  of  the 
nineteenth  century,  we  find  a  large  number  of  new  industries 
created  by  the  demand  for  electric  current  for  lighting.  An 
assistance  also  has  been  given  to  pure  science  by  the  necessity 
for  more  exact  knowledge  on  the  electrical  and  magnetic 
qualities  of  the  materials  employed  in  the  construction  of 
electrical  appliances.  Collaterally,  a  strong  impetus  was 
given  to  the  improvement  of  other  mechanical  machines, 
such  as  the  steam  engine.  Engineers  directed  attention  to 
the  improvement  of  the  dynamo  in  regard  to  its  mechanical 
construction,  and  electricians  provided  new  information,  not 
only  on  the  electrical  theory  of  the  dynamo,  but  on  the 
principles  affording  a  means  for  predetermining  the  results 
of  operation  for  any  particular  design. 

The  result  has  been  the  evolution  of  a  new  machine— viz., 
the  combined  high-speed  steam  engine  and  dynamo— which 
places  in  our  possession  a  most  perfect  and  highly-efficient 
device  for  converting  the  energy  01  high-pressure  steam  into 
electric  energy  in  the  form  of  an  electric  current ;  the  total 
loss  in  conversion  not  exceeding  10  per  cent,  in  large  plants. 


G      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

Simultaneously  with  these  improvements  in  the  generating 
appliances  proceeded  others  for  effecting  the  distribution  and 
use  of  electric  currents,  particularly  in  incandescent  and  arc 
lamps.  Moreover,  it  was  soon  found  that  incandescent  electric 
lighting  lent  itself  very  readily  to  decorative  purposes  in  a 
manner  impossible  for  the  older  illuminants.  Hence  the 
subject  of  "Electric  Lamps  and  Electric  Lighting"  has  not 
only  a  scientific  and  commercial,  but  also  an  artistic  side,  and 
has  considerable  practical  interest  for  every  householder. 

Accordingly,  in  the  following  short  treatise  attention  will 
be  directed  to  the  elucidation  of  facts  which  ought  to  be 
known  by  every  user  of  the  electric  light,  and  the  knowledge 
of  which  will  enable  him  to  understand  something  of  the 
principles  which  underlie  the  art  of  electric  illumination,  and 
to  comprehend  as  well  the  aid  which  it  can  render,  when 
properly  applied,  to  beautify  and  please. 

It  will  be  necessary  to  open  the  whole  of  our  discussion  by 
some  simple  iliustrations  of  the  meaning  of  fundamental  terms. 
Every  science  as  well  as  every  art  has  its  necessary  technical 
terms,  and  even  if  these  words  at  first  sound  strangely,  they  are 
not  therefore  necessarily  difficult  to  understand.  We  are  all 
familiar  with  the  fact  that  electric  illumination  depends  upon 
the  utilisation  of  something  which  we  call  the  electric  current. 
Little  by  little  scientific  research  may  open  up  a  pathway 
towards  a  fuller  understanding  of  the  true  nature  of  an  electric 
current,  but  at  the  present  moment  all  that  we  are  able  to 
say  of  it  is,  that  we  know  what  it  can  do,  how  it  is  produced, 
and  the  manner  in  which  it  can  properly  be  measured.  Two 
principal  facts  connected  with  it  are,  that  when  a  conductor, 
such  as  a  metallic  wire  or  a  carbon  filament,  or  any  other 
material  which  is  capable  of  being  employed  as  a  conductor, 
is  traversed  by  an  electric  current,  heat  is  generated  in  the 
conductor,  and  the  space  round  the  conductor  becomes  capable 
of  influencing  a  magnetic  needle.  These  facts  can  be  simply 


ELECTRIC  MEASUREMENT. 


illustrated  by  passing  an  electric  current  through  an  iron  wire 
(sec-  Fig.  2).  You  will  notice  that  as  the  current  is  gradually- 
increased  the  iron  wire  is  brought  up  from  a  condition  in 
which  it  is  only  slightly  warm  to  one  in  which  it  becomes 
visibly  red  hot  in  the  dark,  and  finally  brilliantly  incandescent. 
At  the  same  time  if  we  explore  the  region  round  about  the  wire 
with  a  suspended  compass  needle,  we  find  that  at  every  point 
in  the  neighbourhood  of  the  wire  the  magnetic  needle  places 
itself,  or  tries  to  place  itself,  in  a  position  perpendicular  to  the 


FIG.  2. — An  iron  wire  W  is  rendered  incandescent  by  an  electric  current 
sent  through  it  from  a  battery  B.  The  magnetic  needle  N  S  held  near  it 
sets  itself  across  the  wire. 

wire.  This  fact,  of  capital  importance,  was  discovered  by 
H.  C.  Oersted  in  1820,  and  in  the  Latin  memoir  in  which  he 
describes  his  epoch-making  discovery  he  employs  the  following 
striking  phrase  to  express  the  behaviour  of  a  magnetic  needle 
to  the  wire  conveying  the  current.  He  says,  "The  electric 
conflict  performs  circles  round  the  wire."  That  state  which 
he  called  the  electric  conflict  round  the  wire,  we  now  in  more 
modern  language  call  the  magnetic  field  embracing  the  con- 
ductor. We  shall  return  in  a  later  lecture  to  this  last  fact. 


8       ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

Meanwhile  I  wish  at  present  to  fasten  your  attention  on  the 
heating  qualities  of  an  electric  current,  and  the  laws  of  that 
heat  production  and  radiation.  The  same  electric  current 
produces  heat  at  very  different  rates  in  different  conductors, 
and  the  quality  of  a  body  in  virtue  of  which  the  electric  current 
produces  heat  in  passing  through  it  is  called  its  electric 
resistance.  If  the  same  electric  current  is  passed  through  con- 
ducting wires  of  similar  dimensions,  but  of  different  materials, 
it  produces  in  them  different  quantities  of  heat  in  the  same 
time.  Before  you  (Fig.  3)  is  a  chain  composed  of  spirals 
of  iron  and  copper  wire.  These  wires  are  each  of  the  same 
length  and  of  the  same  diameter.  Sending  through  this  com- 


FIG.  3. — A  chain  W  composed  of  alternate  spirals  of  copper  C  and  iron  I 
is  traversed  by  an  electric  current  sent  through  it  by  a  battery  B.  The 
iron  links  become  red  hot,  the  copper  links  only  slightly  warm. 

pound  chain  an  electric  current,  we  notice  that  the  iron  wire 
links  are  very  soon  brought  up  to  a  bright  red  heat,  whilst  the 
copper  links,  though  slightly  warm,  are  not  visibly  hot.  We 
have,  therefore,  before  us  an  illustration  of  the  fact  that  a 
current  heats  the  conductor,  but  that  each  conductor  has  a 
specific  quality  called  its  electrical  resistance,  in  virtue  of  which 
the  same  strength  of  current  produces  heat  in  it  at  a  rate 
depending  on  the  nature  of  the  material.  Other  things  being 
equal,  the  bodies  which  are  most  heated  are  said  to  have 
the  highest  resistance. 


ELECTRIC  MEASUREMENT. 


0 


•  It  is  now  necessary  to  notice  the  units  in  which  these  two 
quantities,  namely,  electric  current  and  electric  resistance,  are 
measured.  For  the  sake  of  distinction,  units  of  electric 
quantities  are  named  after  distinguished  men.  We  follow  a 
similar  custom  in  some  respects  in  common  life,  as  when  we 
speak  of  a  "  Gladstone  "  bag  or  a  "  Hansom "  cab,  and 
abbreviate  these  terms  into  a  gladstone  and  a  hansom. 
Primarily  the  distinctive  words  here  used  are  the  names  of 
persons,  but  by  application  and  abbreviation  they  become  the 
names  of  things.  An  electric  current  is  measured  in  terms  of 
a  unit  current  which  is  called  an  ampere,  and  electrical 
resistance  is  measured  in  terms  of  a  unit  which  is  called  an 
ohm,  these  being  respectively  named  after  two  great  investi- 


FIG.  4. — Glass  lantern  trough  containing  two  lead  plates  n  p,  and  a 
solution  of  sugar  of  lead.  When  a  current  from  a  battery  B  is  sent  through 
the  cell  it  deposits  the  lead  in  tufts  on  the  negative  plate  n. 

gators,  Andre  Marie  Ampere  and  Georg  Simon  Ohm.  In 
order  to  understand  the  mode  in  which  an  electric  current 
can  thus  be  defined,  we  must  direct  attention  to  another 
property  of  electric  currents,  namely,  their  power  of  decom- 
posing solutions  of  metallic  salts.  You  are  all  familiar  with 
the  substance  which  is  called  sugar  of  lead,  or,  in  chemical 
language,  acetate  of  lead.  Placing  in  a  small  glass  trough 
a  solution  of  acetate  of  lead  and  two  lead  plates  (see  Fig.  4),  I 
place  the  cell  in  the  electric  lantern  and  project  the  image 
upon  the  screen.  If  an  electric  current  is  passed  through 
the  solution  from  one  lead  plate  to  the  other  it  decomposes 


10     ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

the  solution  of  acetate  of  lead,  extricating  from  the  solution 
molecules  of  lead  and  depositing  them  on  one  of  the  lead 
plates,  and  you  see  the  tufted  crystals  of  lead  being  built  up 
in  frond-like  form  on  the  negative  pole  in  the  cell.  We  might 
employ,  in  preference  to  a  solution  of  acetate  of  lead,  a  solution 
of  nitrate  of  silver,  which  is  the  basis  of  most  marking  inks, 
and  the  same  effect  would  be  seen.  It  was  definitely  proved 
by  Faraday  that  we  might  define  the  strength  of  an  electric 
current  by  the  amount  of  metal  which  it  extricates  from  the 
solution  of  a  metallic  salt  in  one  second,  minute,  or  hour.  The 
Board  of  Trade  Committee  on  Electrical  Standards  have  now 
given  a  definition  of  what  is  to  be  understood  by  an  electric 
current  of  one  ampere  in  the  following  terms :  An  electric 
current  of  one  ampere  is  a  current  which  will  in  one  hour 
extricate  from  a  solution  of  nitrate  of  silver  4-025  grammes 
of  silver.*  Otherwise  we  might  put  it  in  this  manner :  A 
current  of  electricity  is  said  to  have  a  strength  of  one  ampere 
if,  when  passed  through  a  solution  of  nitrate  of  silver,  it 
decomposes  it  and  deposits  on  the  negative  plate  one  ounce  of 
silver  in  very  nearly  seven  hours.  We  are  acquainted  in  the 
laboratory  with  currents  of  electricity  so  small  that  they 
would  take  100,000  years  of  continuous  action  to  deposit 
one  ounce  of  silver,  and  we  are  familiar  in  electric  lighting 
practice  with  currents  great  enough  to  deposit  one  hundred- 
weight of  silver  in  thirty  minutes.  The  simple  experiment 
just  shown  is  the  basis  of  the  whole  art  of  electro-plating. 
Hence,  when  we  speak  later  of  a  current  of  one  ampere,  or  ten 
amperes  you  will  be  able  to  realise  in  thought  precisely  what 
such  a  current  is  able  to  achieve  in  chemical  decomposition. 
It  may  be  convenient  at  this  stage  to  bring  to  your  notice  the 
fact  that  an  8  candle-power  incandescent  lamp  working  at  100 
volts  usually  takes  a  current  of  about  one-third  of  an  ampere, 
a  current  which  would  deposit  by  electro -plating  action  one 
ounce  of  silver  in  about  twenty-one  hours. 

*  28'3495  grammes  =  1  ounce  avoirdupois. 


ELECTRIC  MEASUREMENT. 


11 


We  pass  next  to  consider  another  important  matter,  viz., 
that  of  electric  pressure  or  potential ;  and  we  shall  be  helped 
in  grasping  this  idea  by  considering  the  corresponding  concep- 
tion in  the  case  of  the  flow  of  fluids.  When  a  fluid  such  as 
water  flows  along  a  pipe  it  does  so  in  virtue  of  the  fact  that 
there  is  a  difference  of  pressure  between  different  points  in 
the  pip9,  and  the  water  flows  in  the  pipe  from  the  place  where 
the  pressure  is  greatest  to  the  place  where  the  pressure  is 
least.  On  the  table  before  you  is  a  horizontal  pipe  (Fig.  5) 
which  is  connected  with  a  cistern  of  water,  and  which  delivers 


FIG.  5.— Horizontal  pipe  P,  having  six  vertical  gauge  tubes  attached  to 
it.  The  pipe  P  is  in  connection  with  a  water  cistern  C,  and  when  the  tap 
T  is  shut  the  water  stands  up  at  the  same  level,  A  B,  ia  all  the  tubes. 

water  to  another  receptacle  at  a  lower  level.  In  that  pipe  are 
placed  a  number  of  vertical  glass  tubes  to  enable  us  to  measure 
the  pressure  in  the  pipe  at  any  instant.  The  pressure  at  the 
foot  of  each  gauge  glass  is  exactly  measured  by  the  head  or 
elevation  of  the  water  in  the  vertical  gauge  glass,  and  at  the 
present  moment,  when  the  outlet  from  the  horizontal  pipe  is 
closed,  you  will  notice  that  the  water  in  all  the  gauge  glasses 
stands  up  to  the  same  height  as  the  water  in  the  cistern.,  In 
other  words>  the  pressure  in  the  pipe  is  everywhere  the  same. 


12      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING 

Opening  the  outlet  tap  we  allow  the  water  to  flow  out  from 
the  pipe,  and  you  will  then  observe  that  the  water  sinks  (see 
Fig.  6)  in  each  gauge  glass,  and,  so  far  from  being  noTV  uniform 
in  height,  there  is  seen  to  be  a  regular  fall  in  pressure 
along  the  pipe,  the  gauge  glass  nearest  the  cistern  showing 
the  greatest  pressure,  the  next  one  less,  the  next  one  less 
still,  and  so  on,  the  pressure  in  the  horizontal  pipe  gradually 
diminishing  as  we  proceed  along  towards  the  tap  by  which 
the  water  is  flowing  out.  This  fall  in  pressure  along  the  pipe 
takes  place  in  every  gas  and  water  pipe,  and  is  called  the 


FIG.  6. — Horizontal  pipe  P,  through  which  water  is  flowing  from  a 
cistern  C  to  a  reservoir  R.  When  the  tap  T  is  open  the  water  stands  at 
gradually  decreasing  heights  in  the  pressure  tubes.  The  dotted  line  A  B 
shows  the  hydraulic  gradient. 

hydraulic  gradient  in  the  pipe.  The  flow  of  water  takes 
place  in  virtue  of  this  gradient  of  pressure.  It  will  be  next 
necessary  to  explain  to  you  that  there  is  an  exactly  similar 
phenomenon  in  the  case  of  an  electric  current  in  a  wire,  and 
that  there  is  a  quantity  which  we  may  call  the  electric  pres- 
sure, which  diminishes  in  amount  as  we  proceed  along  the 
wire  when  the  current  is  flowing  in  it.  In  order  to  under- 
stand the  manner  in  which  this  electric  pressure  can  be 


ELECTRIC  MEASUREMENT. 


13 


measured,  a  few  preliminary  experiments  will  be  essential. 
Every  body  which  is  charged  with  electricity  has,  in  virtue  of 
that  charge,  a  certain  electrical  potential,  or  pressure,  as  it  is 
called,  and  electricity  always  tends  to  flow  from  places  of 


FIG.  7. — A  small  Wimshurst  electrical  machine,  having  two  strips  of 
paper  A  and  B  attached  to  its  terminals.  When  the  machine  is  worked 
the  paper  strips  are  drawn  together. 

higher  to  lower  potential,  just  as  water  or  other  fluids  tend 
to  flow  from  places  of  greater  to  less  pressure.  When  two 
bodies  are  at  different  electric  pressures,  or  potentials,  it  is 
found  that  there  is  an  attraction  or  stress  existing  between 


14      ELECTEIC  LAMPS  AND  E'LECTEIC  LIGHTING. 

them,  and  a  tendency  for  them  to  move,  if  possible,  nearer 
together.  If  I  attach  to  the  terminals  of  a  small  electrical 
machine  two  paper  strips,  and  then  charge  those  paper 
strips  to  different  electric  pressures,  we  find  the  strips 


Fm.  8.— Lord  Kelvin's  Multicellular  Electrostatic  Voltmeter. 

The  fixed  plates  or  cells  are  marked  A  in  the  sectional  drawing.  The  movable 

plates  n  are  attached  to  an  axis  S  suspended  by  a  fine  wire  F. 

•are  drawn  together  (see  Fig.  7).  The  difference  of  pressure 
between  these  two  bodies  can  be  exactly  measured  by  the 
mechanical  force  with  which  they  attract  one  another,  or  by 
the  force  required  to  keep  them  apart  by  a  certain  distance. 
This  fact  is  taken  advantage  of  to  construct  many  instruments, 


ELECTRIC  MEASUREMENT. 


15 


which  are  called  electric  pressure-measuring  instruments,  or 
voltmeters.  One  of  the  most  valuable  of  these  is  the  electro- 
static voltmeter,  invented  by  Lord  Kelvin.  It  consists  (see 
Fig.  8)  of  a  series  of  fixed  plates,  which  are  called  cells,  and 
suspended  between  these  are  a  number  of  movable  plates,  all 
attached  to  a  common  axis,  this  axis  being  suspended  by  a 
very  fine  wire.  The  suspended  plates  are  so  arranged  that, 
when  at  a  different  electric  pressure  or  potential  from  the  fixed 
plates,  they  are  attracted  in  between  them,  and  the  movement 
of  the  suspended  plates  is  resisted  by  the  torsional  elasticity 


FIG.  9. — A  battery  C,  which  in  the  actual  experiment  consists  of  50  cells, 
sends  a  current  through  a  wire  WW.  By  means  of  a  pair  of  contact 
pieces  A  B,  the  terminals  of  an  electrostatic  voltmeter  V  are  connected  to 
various  points  on  the  wire  W  W,  and  the  fall  in  electric  pressure  along  it 
is  thus  measured. 

of  the  suspending  wire.  The  extent  to  which  they  thus 
move  can  be  measured  by  an  indicating  needle  fastened  to  the 
movable  plates.  If  the  fixed  and  movable  plates  in  this  instru- 
ment are  brought  to  different  electric  pressures,  or  different 
electric  potentials,  they  will  be  attracted  towards  one  another, 
and  we  then  have  an  instrument  which  can  be  converted  by 
proper  graduation  into  an  electric  pressure-measuring  instru- 
ment. Furnished  with  such  an  appliance  we  can  now  explore 
the  change  in  pressure  down  a  wire  through  which  the 
electric  current  is  flowing.  Through  this  manganese-steel 
wire  we  are  now  passing  a  current  of  electricity.  One  ter- 
minal of  the  voltmeter,  namely,  that  connected  to  the  fixed 


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ELECTRIC  MEASUREMENT. 


any  wte  or  «awbetar 
of  which  is  called  one  obi 

thefint  dear  definition  of  Hie 
rat,  electric  pre^re,  and  electric 


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18      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

we  shall  see  in  the  next  lecture,  of  a  fine  carbon  thread, 
or  filament,  which,  as  usually  made,  is  traversed  by  a 
current  of  about  two-thirds  of  an  ampere,  when  that  con- 
ductor is  one  which  is  suitable  for  a  IG-candle-power  lamp 
worked  at  the  usual  pressure  of  100  volts.  The  electric 
supply  companies  bring  into  our  houses  two  wires,  between 
which  they  are  constantly  engaged  in  keeping  an  electric  pres- 
sure difference  of  100  volts  or  200  volts,  or  thereabouts.  If, 
therefore,  the  terminals  of  a  lamp  are  connected  to  these  two 
supply  wires,  the  ends  of  the  carbon  filament  are  exposed  to 
an  electric  pressure  of  100  volts.  The  electric  resistance  of 
that  carbon,  when  incandescent,  is  therefore  expressed  in  ohms 
by  dividing  the  number  expressing  the  pressure  difference  in 
volts  by  the  number  defining  the  current  in  amperes ;  hence 
it  is  the  quotient  of  100  by  two-thirds,  or  150  ohms. 

Another  fundamental  law  in  connection  with  the  flow  of  an 
electric  current  in  conductors  was  enunciated  by  Mr.  Joule  in 
1841,  and  is  called  Joule's  law.  It  is  thus  stated :  If  a  cur- 
rent flows  through  an  electric  conductor,  the  heat  produced  in 
that  conductor  per  second  is  proportional  to  the  product  of 
the  square  of  the  current  strength  as  measured  in  amperes 
and  the  resistance  of  the  conductor  measured  in  ohms.  Joule 
deduced  this  law  from  elaborately  careful  experiments  made 
on  the  quantity  of  heat  produced  in  a  certain  wire  when  tra- 
versed by  an  electric  current,  that  wire  being  immersed  in 
water.  His  experimental  procedure  was  as  follows : — He 
immersed  a  wire,  formed  into  a  spiral,  in  a  vessel  of  water  so 
protected  as  not  to  be  able  to  lose  heat  from  the  outside.  He 
then  passed  measured  currents  of  electricity  through  the 
spiral,  and  observed  with  delicate  thermometers  the  rise  of 
temperature  of  the  water  in  a  stated  time.  The  whole  of  the 
energy  which  is  thus  being  spent  in  the  wire  is  converted  into 
heat,  and  that  heat  is  employed  in  raising  the  temperature  of 
the  water.  If  by  suitable  means  we  prevent  the  loss  of  heat 


ELECTRIC  MEASUREMENT.  19 

from  the  containing  vessel,  or  otherwise  take  it  into  account, 
and  if  we  try  this  experiment  with  currents  of  two  different 
strengths,  say  of  one  ampere  and  two  amperes,  it  will  be 
found  that,  if  the  resistance  of  the  conductor  remains  the  same, 
the  heat  generated  in  a  given  time  by  the  current  of  two 
amperes  will  be  four  times  as  great  as  the  heat  generated  in 
the  same  time  by  a  current  of  one  ampere,  and  in  like  manner 
a  current  of  three  amperes  would  generate  nine  times  as  much 
heat  as  a  current  of  one  ampere,  always  provided  that  the 
resistance  of  the  wire  is  not  sensibly  changed  when  the  cur- 
rent is  altered.  A  little  consideration  of  the  law  of  Joule 
and  the  law  of  Ohm,  when  taken  together,  will  show  you 
that,  since  the  total  amount  of  heat  produced  per  second  by  a 
current  of  a  given  magnitude  is  proportional  to  the  products 
of  the  numbers  representing  the  resistance  of  the  circuit 
in  ohms,  and  the  square  of  the  strength  of  the  current 
measured  in  amperes  flowing  through  it ;  and,  since  the 
product  of  the  value  of  the  resistance  of  the  circuit  in 
ohms  and  the  current  strength  in  amperes  is  numerically 
equal  to  the  difference  of  pressure  between  the  two  ends 
of  the  conductor,  it  follows  that  the  total  rate  at  which 
energy  is  being  expended  in  any  conductor  to  produce  heat 
when  a  current  of  electricity  is  flowing  through  it  is  measured 
by  the  numerical  product  of  the  strength  of  that  current 
in  amperes,  and  the  pressure  difference  between  the  terminals 
of  that  conductor  measured  in  volts.  If  we  apply  this  rule 
to  the  case  of  an  electric  lamp  we  find  that,  in  order  to 
measure  the  total  rate  at  which  energy  is  being  transformed 
into  heat  and  light  in  an  incandescent  electric  lamp,  we 
have  to  measure,  in  the  first  place,  the  current  passing 
through  it  in  amperes  and  the  pressure  difference  between 
the  terminals  of  the  lamp  in  volts,  and  the  product  then  gives 
us,  in  certain  units,  which  are  called  watts,  the  rate  at  which 
energy  is  being  dissipated  or  converted  into  heat  in  the  carbon 
filament.  Thus,  for  example,  if  a  lamp  which  takes  two- 

C  3 


20      ELECTRIC  LAMPS  AND  ELECTEIC  LIGHTING. 

thirds  of  an  ampere  is  placed  upon  a  circuit  having  a 
pressure  difference  of  100  volts,  the  product  of  100  and  two- 
thirds  being  66,  the  lamp  would  be  taking  66  watts,  and 
this  is  the  measure  of  the  rate  at  which  energy  is  being 
supplied  to  the  lamp,  and  converted  by  it  into  light  and  heat. 

It  is  important  that  you  should  possess  a  very  clear  con- 
ception of  the  exact  meaning  attached  to  the  watt  as  a  unit  of 
power.  If  a  weight  of  one  pound  is  lifted  one  foot  high,  the 
amount  of  exertion  or  work  required  to  raise  this  weight  is, 
in  engineering  language,  called  one  foot-pound  of  work.  If 
550  pounds,  or  nearly  one-quarter  of  a  ton,  are  so  lifted  one 
foot  high,  the  work  done  is  called  550  foot-pounds.  Imagine 
this  last  exertion  made  in  one  second,  and  repeated  every 
second ;  the  rate  at  winch  work  is  being  done,  or  exertion  made, 
will  be  equal  to  that  which  is  called  one  horse-power.  An 
ordinary  man  might  for  some  time  keep  on  lifting  55  pounds, 
say  the  weight  of  a  good-sized  full  portmanteau,  one  foot 
high  per  second,  and  he  would  then  be  doing  one-tenth 
of  a  horse-power.  The  work  done  in  lifting  one  pound 
about  nine  inches  high,  or  more  nearly  0-7373  of  a  foot, 
has  been  selected  as  a  unit  of  work,  and  is  called  one 
joule,  and,  if  this  work  is  repeated  every  second,  this  rate 
of  doing  work,  or  making  exertion,  is  called  one  watt.  It 
is  necessary  to  notice  carefully  that  a  watt  is  a  unit  rate  of 
doing  work  and  not  a  unit  of  work.  If  the  pound  weight  is 
lifted  against  gravity  nine  inches  high  slowly  or  quickly  the 
work  done  is  the  same,  viz.,  one  joule.  If  it  is  lifted  nine 
inches  high  in  one  second  this  rate  of  doing  work  is  called  one 
watt.  If  it  is  lifted  nine  inches  high  in  half  a  second,  the  rate 
of  doing  work  is  doubled,  and  is  then  two  watts.  Hence 
it  will  be  seen  that  one  watt  is  a  unit  of  power,  which  is 
the  y^e-th  part  of  a  horse-power.  The  reason  for  choosing 
such  a  small  unit  is  that  it  gives  convenient  numbers  in  which 
to  measure  the  rate  at  which  work  is  done  in  such  energy 


ELECTRIC  MEASUREMENT.  21 

transforming  agents  as  incandescent  lamps.  When  a  larger 
unit  is  necessary,  it  is  usual  to  employ  the  kilowatt,  which  is 
equal  to  1,000  watts,  as  a  unit  of  power,  and  it  is  ohvious 
that  a  kilowatt  is  equal  to  about  one  and  one-third  of  a  horse- 
power. Accordingly,  such  a  lamp  as  we  have  above  assumed 
dissipates  energy  at  a  rate  which  would  require  one  horse-power 
to  maintain  eleven  or  twelve  lamps,  and  the  rate  at  which 
physical  energy  has  to  be  supplied  to  a  16-candle-power  glow 
lamp  to  keep  it  at  full  candle-power  is  almost  as  great  as  the 
maximum  rate  at  which  a  strong  man  can  do  work. 

Having  prepared  the  way  for  further  explanations  by  these 
elementary  definitions,  we  must  now  proceed  to  examine  more 
carefully  what  happens  when  an  electric  current  is  sent 
through  a  conductor,  and  the  temperature  of  that  conductor  is 
allowed  to  rise.  Coming  back  to  our  initial  experiment  with 
the  iron  wire  heated  by  an  electric  current,  we  must  note  that 
there  are  three  ways  in  which  the  energy  supplied  to  that  wire 
is  being  dissipated.  In  the  first  place,  it  is  dissipated  by  con- 
tact with  the  cold  air  around  it.  The  air  molecules  are  con- 
tinually beating  against  the  hot  wire,  coming  up  to  it  cool, 
taking  energy  from  it  to  heat  themselves,  and  going  away 
warm,  and  this  process  of  carrying  off  of  heat  by  the  air  mole- 
cules is  called  convection.  In  the  next  place,  the  wire  loses 
heat  by  conduction  out  at  the  ends  ;  the  warm  wire  is  held 
in  cool  metallic  supports,  which  are  so  large  and  such  good 
conductors  that  they  are  not  sensibly  heated  by  the  current. 
Hence,  part  of  the  heat  of  the  wire  is  carried  away  by  them, 
and  if  you  examine  the  red-hot  wire  you  will  find  that  it  is 
cooler  at  the  ends  than  it  is  in  the  middle,  being  not  quite  so 
brilliantly  incandescent  close  up  to  the  clamps  as  it  is  in  the 
centre.  Then,  furthermore,  the  wire  is  losing  energy  by  a 
process  called  radiation.  It  is  imparting  energy  to  the  ether, 
and  sending  out  waves  into  this  ether  which  represent  an 
energy  transformation,  some  of  these  waves  being  of  such  a 


22      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


character  that  they  can  affect  the  retina  of  the  eye,  and  con- 
stitute what  we  call  light ;  but  by  far  the  larger  quantity  of 
that  wave  energy  is  not  capable  of  affecting  the  retina  of  the 
eye,  and  is  called  the  non-luminous  radiation.  The  luminous 
radiation,  as  we  shall  see  later,  in  this  case  is  only  about 
one  or  two  per  cent,  of  the  total  radiation.  Let  us  follow,  then, 
the  changes  that  take  place  as  any  conductor,  say  a  wire  or 
thin  carbon  rod,  is  gradually  heated  to  incandescence.  Prof. 
Draper,  as  far  back  as  1847,  carried  out  such  a  series 
of  experiments  with  a  platinum  wire,  which  he  heated  up 
gradually  by  an  electric  current,  measuring  at  each  stage  the 
total  amount  of  energy  which  was  thrown  out  from  the  wire 
in  the  form  of  heat  and  the  amount  of  energy  thrown  out 
from  the  wire  in  the  form  of  luminous  radiation  or  light. 
Before  you  is  a  table  of  Draper's  results.  His  figures  are  in 
certain  arbitrary  units. 

Table  of  Draper's  Experimental  Results  (in  1847)  on  the  Incan- 
descence of  Platinum  Wire. 


["emp'rature 
of  the  Wire  in 
Centigrade 
degrees. 

The  Heat 
given  out  by 
the  Wire. 

The  Light 
radiated  by 
the  Wire. 

Remarks. 

525°C 

practically  nil 

Just  visible  in  the  dark. 

527°C 

0-87  units 



590°C 

1-10 

Spectrum  visible  to  line  E. 

653°C 

150 

F 

»  j              5  >             »»•*•• 

718°C 

180 

,,F-G. 

782°C 

2-80 

»                 i»                »»      Gr. 

910°C 

370 

Full  spectrum  visible. 

1038CC 

6-80 

0-39  units 

.... 

1100°C 

8-60 

0-62 

.... 

11(>6°C 

10-0 

173 

Spectrum  to  H 

1230°C 

12-5 

292 

.... 

1293°C 

155 

4-40 

.... 

1367°C 

... 

7-24 

.... 

1421°C 

... 

12-34 

.... 

The  above  amounts  of  heat  and  light  are  given  in  arbitrary  units.     The 
letters  E,  F,  G,  refer  to  the  fixed  lines  in  the  solar  spectrum. 


ELECTRIC  MEASUREMENT.  23 

Let  us,  however,  follow  the  whole  progress  of  the  pheno- 
mena experimentally  by  the  employment  of  an  incandescent 
lamp.  Before  me  on  the  table  is  a  carbon  glow  lamp,  having 
a  straight  filament  of  carbon  as  the  conductor  to  be  rendered 
incandescent.  By  means  of  a  lens  we  can  project  an  image  of 
the  glowing  carbon  upon  a  screen,  and  by  means  of  a  prism 
we  can  expand  that  linear  optical  image  into  a  prismatic  spec- 
trum, or  rainbow  strip,  and  notice  the  gradual  changes  which 
occur  as  the  carbon  is  heated  up  from  its  lowest  temperature  to 
the  highest  incandescence  it  will  safely  bear.  By  passing  a  care- 
fully graduated  current  through  the  lamp,  we  find  out  that  at 
a  certain  point  the  carbon  just  begins  to  be  visible  in  the  dark. 
It  has  sometimes  been  inferred,  as  a  deduction  from  Draper's 
experiments,  and  was,  indeed,  stated  by  him,  that  all  bodies 
begin  to  be  visibly  red  hot  in  the  dark  at  the  same  tempera- 
ture, namely,  at  about  525°C.  This,  however,  is  certainly 
not  the  fact.  It  has  been  shown  by  Weber,  Bottomley,  and 
others,  as  the  result  of  careful  experiments  on  carbon  fila- 
ments and  platinum  wires,  that  bodies  with  a  black,  sooty 
surface  have  to  be  brought  up  to  a  higher  temperature  than 
bodies  with  a  bright  metallic  surface  before  they  begin  to  be 
visible  in  complete  darkness.  The  crucial  experiment  has 
been  made  by  J.  T.  Bottomley  of  taking  two  platinum  wires, 
one  of  which  is  made  to  have  a  dull,  sooty  surface  by  coating 
it  with  the  finest  possible  coating  of  lampblack,  the  other 
being  highly  polished.  If  these  are  placed  in  closed  glass 
tubes  from  which  the  air  is  exhausted,  and  electric  currents 
passed  through  the  wires  so  as  to  heat  them,  it  will  be 
found,  on  carefully  increasing  the  current  and  examining  the 
wires  in  complete  darkness,  that  the  wire  which  has  a  bright 
metallic  surface  becomes  visible  in  the  dark  at  a  lower 
temperature  than  the  wire  which  has  a  dull,  sooty  surface. 
In  Mr.  Bottomley's  experiments  the  temperature  of  the 
two  wires  was  obtained  from  their  electrical  resistance ; 
this  last  having  been  carefully  measured  at  various  tempera- 


24      ELECTING  LAMPS  AND  ELECTRIC  LIGHTING. 

tures  previously.  This  and  other  experiments  show  that 
it  is  not  strictly  true  that  all  bodies  become  luminous  in 
the  dark  at  the  same  temperature.  But  approximately  we 
may  say  that  most  bodies,  such  as  metals  and  carbons,  when 
heated  to  600°  Centigrade,  begin  to  give  out  radiation  which 
is  capable  of  affecting  the  eye  as  dull  red  light.  Returning 
to  our  lamp,  let  us  gradually  increase  the  current  through 
the  carbon  conductor  of  this  Bernstein  lamp,  whilst  at  the 
same  time  we  project  the  image  of  the  straight  carbon  upon 
the  screen,  and  examine  it  with  a  prism.  As  the  current  is 
gradually  increased  the  carbon  gives  off  radiation  which  is 
first  entirely  non- luminous,  and  which,  though  not  capable  of 
affecting  our  eyes,  can  be  detected  by  a  very  delicate  thermo- 
meter or  other  suitable  instrumental  means.  As  the  tem- 
perature of  the  wire  is  increased  to  a  higher  point,  radiation 
makes  its  appearance  which  is  capable  of  affecting  the  retina 
of  the  eye.  It  is  sometimes  stated  that  the  first  colour  which 
makes  its  appearance  is  red,  but  careful  observation  shows  that 
the  light  which  impresses  the  eye  first,  and  which  most 
easily  stimulates  the  retina,  is  a  greyish-green,  which  occupies 
in  the  spectrum  the  position  of  maximum  brightness.  If  the 
temperature  of  the  conductor  is  still  further  increased,  we  see 
that  the  prism  shows  us  that  red  and  yellow  rays  have  made 
their  appearance  also,  and  a  short  spectrum  is  projected  upon 
the  screen  in  which  the  red,  yellow,  and  green  rays  are  clearly 
visible.  Increasing  still  more  the  temperature  of  the  conductor 
by  passing  a  stronger  current  through  it,  we  notice  that  the 
spectrum  lengthens,  greenish -blue,  blue,  and  violet  rays 
successively  making  their  appearance,  and  are  added  to  the 
others,  whilst  the  previously  existing  green  and  red  rays,  and 
especially  the  green  and  yellow,  are  much  strengthened.  The 
spectrum,  therefore,  exhibits  a  growth,  which  growth  consists 
in  the  addition  of  more  and  more  refrangible  rays,  or  rays 
towards  the  violet  end  of  the  spectrum,  whilst  at  the  same 
time  a  gradual  increase  in  the  intensity  or  luminosity  of  all 


UNIVERSITY 


ELECTRIC  MEASUREMENT. 


the  rays  takes  place,  so  that,  when  finally  we  have  the  com- 
plete spectrum  exhibited  on  the  screen,  the  conductor  is  found 
on  examination  to  be  in  a  state  of  brilliant  incandescence, 
throwing  out  white  light.  We  can,  therefore,  by  the  assist- 
ance of  the  prism,  watch  the  gradual  progress  in  the  emission 
of  radiation  from  the  carbon  conductor,  which  is  capable  of 
affecting  the  eyes;  but  experiments  can  be  made  which,  at  the 
same  time,  show  that  non-luminous  or  invisible  radiation  is 
being  sent  out  from  the  conductor,  and  that  this,  at  all  stages 
of  the  increase  in  luminosity  of  the  visible  spectrum,  is  also 
increased  in  a  certain  proportion.  Broadly  speaking,  therefore, 
as  we  increase  the  temperature  of  any  body,  it  first  begins  by 
emitting  rays  which  are  not  capable  of  affecting  the  eye,  but 
finally,  as  the  temperature  is  carried  up  to  a  higher  and 
higher  point,  it  emits  successively  rays  which  are  capable  of 
affecting  the  optic  nerve ;  and  in  the  end,  when  a  temperature 
of  1,600  or  1,700°C.  is  reached,  we  have  a  total  radiation  from 
the  conductor,  which  affects  the  eye  as  white  light.  The 
annexed  table  shows  approximately  the  character  of  the  light 
emitted  from  bodies  when  raised  to  different  temperatures  : — 
Radiation  from  .Bodies  at  different  Temperatures. 


Centigrade. 


Bodies  begin  to  be  just  visible  in  the  dark  at  about ... 

Dull  red  heat  at 

Dull  cherry-red  heat  at  ... 

Full  cherry-red  at  

Melting  point  of  silver  at 

Clear  red  heat  at 

Melting  point  of  gold  at 

White  cast-iron  melts  at... 

Orange-red  heat  at  

Bright  orange  heat  at 

White  heat  at       

Steel  melts  at 

Bright  white  heat  at        

Dazzling  white  heat  at    ... 
Palladium  melts  at 
Wrought-iron  melts  at    ... 
Platinum  melts  at 
Iridiurn  melts  at  . 


400° 

700° 

800° 

900° 

945° 

1,000° 

1,045° 

1,040° 

1,100° 

1,200° 

1,300° 

1,300° 

1,400° 

1,500° 

1,500° 

1,600° 

1,775° 

1950° 


26      ELECTEIC  LAMPS  AND  ELECTRIC  LIGHTING. 


Eeturning  again  to  our  incandescent  wire,  it  must  be  noted 
that,  of  the  total  radiation  sent  out  by  such  a  hot  body,  only 
a  small  fraction  of  that  radiant  energy  is  capable  of  affecting 
the  eye,  and  making  its  impression  upon  us  as  light.  By  far 
the  larger  proportion  of  radiation  from  most  incandescent 
bodies  is  non-luminous  radiation,  and  makes  no  impression  at 
all  upon  the  organ  of  sight.  The  proportion  of  luminous  to 
total  radiation  from  various  sources  has  been  measured  by 
different  observers,  with  the  following  results  : — 

Proportion  of  Luminous  to  Non-Luminous  Radiation  in 
Various  Sources  of  Light. 


Source. 

Luminous 
Eadiation. 

Non-Luminous 
Radiation. 

Red-hotwire  ... 
Hydrogen  flame 

practically  nil. 
ditto. 

100     per  cent. 
100 

Oil  flame 

3       per  cent. 

97 

Gas  flame 

4 

96 

White-hot  wire 

4-5 

95-5 

Electric  glow  lamp     . 

3  to  7 

95 

Arc  lamp 

5  to  15     , 

90 

Sunlight 

34 

66 

The  above  table  shows  us,  then,  the  percentage  of  luminous 
to  non-luminous  radiation  in  these  various  sources  of  light. 
The  luminous  efficiency  of  any  illuminant  is  denned  as  the 
fraction,  expressed  as  a  percentage,  which  the  luminous 
radiation  is  of  the  total  radiation.  We  may  also  express,  in 
the  unit  previously  denned,  called  the  watt,  the  rate  at  which 
energy  is  being  expended  in  any  illuminant  to  produce  an 
illuminating  power  equal  to  that  of  one  candle,  and  coupling 
this  with  the  known  luminous  efficiency  of  the  illuminant, 
we  obtain  two  numbers  which  precisely  express  the  energy 
consumption  of  the  agent,  and  that  which  may  be  called  its 
efficiency  as  a  translating  device  for  converting  energy  from 
one  form,  whether  electrical  or  chemical,  into  another  form — 
namely,  eye-affecting  wave-motion  in  the  ether.  Below  are 


ELECTE10  MEASUREMENT. 


collected    together    these    numbers    for  various    sources   of 

light  :  — 

Efficiency  of  Various  Sources  of  Light. 


Source  of  Light. 

Power  Consumption 
in  Watts  required  to 
produce  a   light   of 
one  Candle. 

Ratio  of  Luminous  to 
total  Radiation,  or 
Luminous  Efficiency. 

Candle  
Oil  lamp 
Petroleum  lamp 
Argand  gas  lamp 
Electric  glow  lamp     .  .  . 
Electric  arc     
Magnesium  wire 
Electric  discharge     in 
rarefied  (fasos 

86       watts 
57 
42-8 
68-8 

3£ 
0-8 

2  to    3  per  cent. 
3 
3 
4 
3  to    7 
5  to  15 
15 

33 

One  great  problem  awaiting  solution  in  the  future  is  the 
discovery  of  a  source  of  artificial  illumination  which  is, 
relatively  speaking,  much  more  efficient  than  those  which  are 
at  present  available ;  that  is  to  say,  of  a  method  which  will 
convert  a  much  greater  proportion  of  the  energy  transformed, 
into  radiation  strictly  limited  to  that  which  can  affect  the 
eye ;  whether  that  energy  be  electrical  energy  in  the  form 
of  an  electric  current,  or  whether  it  be  chemical  potential 
energy  associated  with  materials  to  be  combined.  It  appears 
probable  that  the  solution  of  this  problem  rests  in  the  develop- 
ment of  phosphorescence  into  a  practical  method  for  yielding 
light.  By  the  employment  of  apparatus  of  extraordinary 
delicacy,  Prof.  S.  P.  Langley  and  Mr.  Very,  in  America,  have 
succeeded  in  determining  the  luminous  efficiency  of  the  light 
emitted  by  the  Cuban  fire-fly.  They  find  that  in  this  natural 
light  the  whole  energy  of  radiation  is  comprised  within  the 
limits  of  the  visible  spectrum,  and,  what  is  more  important, 
chiefly  within  the  limits  of  the  green  and  greenish-yellow  rays, 
which  are  especially  important  for  the  purposes  of  vision.  These 
experiments  show  that  the  light  emitted  by  this  insect  is, 


28      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

indeed,  light  without  heat,  and  that  its  luminous  efficiency 
is  not  far  below  100  per  cent.  In  our  artificial  production  of 
light  we  have  much  lee-way  to  make  up  before  we  can  rival 
the  efficiency  of  the  glow-worm  and  fire-fly. 

Before,  however,  we  can  discuss  this  matter  further,  it  will 
be  necessary  to  turn  our  attention  a  little  more  at  length  to 
the  subject  of  photometry,  or  the  comparison  of  different 
sources  of  illumination  in  regard  to  their  light-giving  quali- 
ties. This  is  a  part  of  our  subject  which,  we  may  say  at 
once,  is  in  a  much  less  satisfactory  condition,  as  regards  exact 
measurement,  than  the  other  practical  electrical  measurements 
to  which  reference  has  been  made.  The  various  light-giving 
bodies  which  we  know  and  use,  such  as  the  sun,  candle,  gas 
lamp,  incandescent  electric  lamp,  electric  arc  lamp,  &c.,  are 
bodies  which  are  at  very  different  temperatures,  and  they  emit 
light,  therefore,  of  very  different  composition.  Every  ray  of 
light  which  has  a  pure  and  simple  ray  is  characterised  by  two 
qualities — first,  its  wave  length,  which  determines  the  colour 
impression  it  makes  on  the  organ  of  vision ;  and,  secondly,  the 
amplitude  of  that  wave  motion,  which  determines  its  luminous 
intensity. 

It  may  fairly  be  assumed  that  the  reader  is  familiar 
with  the  fact  that  all  optical  research  has  indicated  the  fact 
that  what  we  call  light  is  a  disturbance  of  some  kind  taking 
place  in  a  universal  medium  called  the  ether.  We  do  not 
know  precisely  the  nature  of  that  medium  or  that  disturbance, 
but  crucial  experiment  shows  that,  when  a  ray  of  light  is 
passing  through  space,  some  change  is  being  repeated  very 
rapidly  at  every  point  in  its  path,  and  that  this  disturbance 
travels  out  from  any  point  with  a  velocity  which  is,  approxi- 
mately, 186,000  miles  a  second;  that  is,  it  moves  nearly 
one  foot  in  the  thousand-millionth  part  of  a  second.  This 
disturbance  is  a  wave  motion — that  is  to  say,  at  points  in 


ELECTRIC  MEASUREMENT. 


29 


space  separated  by  certain  intervals  similar  motions  or  actions 
are  taking  place  in  a  periodic  manner  at  the  same  time,  and 
the  distance  between  two  points  in  space  at  which  similar 
actions  or  movements  are  taking  place  is  called  a  wave  length. 
We  are  familiar  with  such  wave  motion  in  the  case  of  sound 
and  the  surface  waves  of  water.  By  optical  processes  the 
wave  lengths  of  light  can  be  measured,  and  they  have  the 
values  given  in  the  table  below.  The  unit  of  length  in 
which  these  wave  lengths  are  expressed  is  the  one-hundred- 
millionth  part  of  a  centimetre. 

Tables  of  Colour*  and  Wave  Lengths  of  Light. 


Colour  of  the  Light. 

Wave  Length. 

Red 

6  800 

Orange-red 

6  550 

5,950 

Yellow 

5  680 

Yellow-green 

5  370 

Pure  green  .  . 

5,200 

Green-blue              .  . 

4  910 

Cyan-blue 

4700 

Pure  blue 

4,580 

Violet-blue                                                               .  . 

4410 

Violet           

4,330 

We  may  call  to  mind  the  fact,  that  in  music  the  length  of 
the  air  wave  producing  any  note  is  exactly  half  the  length  of 
the  wave  corresponding  to  the  note  an  octave  below. 

It  is  evident  from  the  above  figures  that  the  eye  is  sensitive 
to  about  an  octave  of  colour.  It  may  assist  the  imagination  if 
it  is  here  mentioned  that  the  wave  length  of  green  or  greenish- 
blue  light  is  about  l/50,000th  part  of  an  inch.  This  is  not 
by  any  means  an  inconceivably  small  length.  Gold  leaf  may 
be  beaten  out  so  fine  that  the  thickness  of  a  single  sheet 
is  only  l/300,000th  part  of  an  inch,  or  something  like  one- 
sixth  part  of  the  length  of  a  wave  length  of  green  light. 


30      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

In  pure  white  light  we  may  have  rays  of  all  these  wave 
lengths  present,  together  with  any  or  all  the  intermediate 
ones  whose  wave  lengths  have  not  been  given.  Each  different 
source  of  light  emits  in  general  a  great  group  of  rays  of 
many  different  wave  lengths,  and  each  particular  ray  may 
be  present  in  varying  intensity  or  brightness.  The  proportion 
in  which  the  different  rays  exist  in  any  source,  and  the  relative 
intensity  which  these  respective  rays  have,  is  not  the  same 
for  different  sources  of  light.  Hence  the  radiation  from  any 
illuminant  is  in  general  a  very  complex  thing,  and  the  effect 
which  is  produced  when  it  falls  on  our  eyes  is  a  resultant 
or  joint  effect  due  to  all  the  several  individual  rays  that  exist 
in  that  light.  We  have  in  the  prism  a  means  of  analysing 
any  compound  ray  by  separating  out  the  individual  light 
rays  which  compose  it  in  a  fan-like  form,  so  that  we  can 
detect  which  rays  are  present  and  which  are  absent,  and 
measure  also  their  relative  brightness  or  luminosity.  We  did 
this  just  now  when  we  expanded  the  linear  image  of  our  in- 
candescent carbon  filament  into  a  broad  many-coloured  band 
called  a  spectrum.  Two  lights  are  said  to  be  similar  when 
their  spectra  can  be  made  to  have  equal  brightness  in  all 
their  corresponding  parts.  That  is  to  say,  if  the  yellow  ray 
in  one  spectrum  is  made  equal  in  brightness  to  the  yellow  in 
the  other,  then  the  green,  blue  and  red  in  the  two  spectra  are 
also  of  equal  brightness.  A  very  little  investigation  shows  us, 
however,  that  different  sources  of  light  are  not  similar.  I 
throw  upon  the  screen  the  spectrum  of  the  light  given  by  the 
carbon  of  an  incandescent  lamp,  and  also  throw  upon  the 
same  screen  another  spectrum  just  above  the  first,  which  is 
formed  from  the  light  of  an  electric  arc  lamp.  These  spectra 
are  formed  by  exactly  similar  prisms  placed  symmetrically 
with  regard  to  the  screen,  and  you  will  notice  that  not  only 
are  similarly  coloured  rays  in  the  two  spectra  not  equally 
bright,  but  you  will  note  the  especial  richness  of  the  spectrum 
of  the  arc  light  in  violet  rays.  We  can  so  adjust  these  two 


ELECTRIC  MEASUREMENT.  31 

spectra  that  they  become  exactly  equal  in  brightness  or 
luminosity  for  any  particular  ray,  say  the  yellow — that  is  to 
say,  we  can  so  weaken  the  light  of  the  arc  lamp  that 
its  spectrum  in  the  yellow  is  exactly  equal  in  brightness  to 
the  spectrum  of  the  incandescent  lamp  in  the  yellow.  When 
this  is  done,  we  find  that  the  arc  lamp  spectrum  is  very 
much  brighter  in  the  violet,  green,  and  blue  than  that  of 
the  incandescent  lamp  spectrum,  but  that  it  is  not  so 
bright  in  the  red.  It  is  clear  therefore,  that  these  two 
lights  are  dissimilar  in  quality,  and  before  we  can  speak 
of  the  comparison  of  two  lights,  we  must  understand  dis- 
tinctly what  we  are  going  to  compare. 

Broadly  speaking,  the  two  great  purposes  for  which 
we  require  artificial  light  are  to  discriminate  form  and 
to  discriminate  colour  difference.  With  regard  to  the 
discrimination  of  form,  it  is  not  necessary  that  the 
light  which  is  employed  should  possess  rays  of  more 
than  one  colour  or  wave  length.  Such  a  light  is 
called  mono-chromatic.  If  I  illuminate  this  room  with 
light  which  is  wholly  yellow  or  wholly  red,  by  burning 
metallic  sodium  in  the  flame  of  a  spirit  lamp,  or,  in 
a  similar  manner,  by  burning  some  nitrate  of  strontium, 
which  is  the  basis  of  red  fire,  we  find  that  we  are 
able  to  discriminate  the  form  of  surrounding  objects, 
but  that  their  normal  colour  differences  have  vanished.  I 
need  not  spend  much  time  in  reminding  you  that  what 
we  call  the  colour  of  different  objects  is  only,  a  physical 
difference  in  their  surface  or  substance  which  enables  them 
to  select  certain  rays  out  of  a  compound  bundle  of  rays 
falling  upon  them  for  reflection,  and  to  absorb  the  rest. 
Thus,  a  ripe  cherry  is  red  because,  out  of  the  whole  col- 
lection of  rays  of  light  falling  upon  it,  it  sends  back  to 
our  eyes  by  preference  a  large  proportion  of  the  red,  yellow, 
and  green  rays,  but  absorbs  some  violet  and  blue  ;  and  the 


32      ELECTEIG  LAMPS  AND  ELECTRIC  LIGHTING. 

leaves  of  the  cherry  tree  are  green  for  the  reason  that 
they  absorb  a  large  proportion  of  rays  other  than  green, 
but  reflect  more  copiously  these  last.  This  selective  re- 
flection is,  however,  due  to  the  absorption  or  diminution 
in  intensity  of  certain  rays  in  the  compound  light  falling 
on  the  body,  and  which,  passing  through  the  surface  of  the 
reflecting  body,  is  internally  reflected,  and  then  is  returned 
back  through  the  surface,  having  in  the  double  transit 
suffered  a  deprivation  or  weakening  of  certain  constituent  rays. 
Hence,  the  colour  of  a  body  is  dependent  upon  the  character 
of  the  radiation  which  falls  upon  it,  and  if  we  illuminate  a 
group  of  coloured  bodies,  first  by  light  which  is  purely  red, 
and  then  by  light  which  is  purely  green,  we  find  that  in 
these  two  cases  the  apparent  colour  of  the  bodies  is  totally 
different. 

These  facts  we  are  able  to  illustrate  before  you  in  a 
very  simple  manner  by  throwing  some  red  light,  produced 
by  passing  the  light  from  the  electric  lantern  through  a  pure 
ruby-red  glass,  upon  a  group  of  coloured  ribbons  or  pieces 
of  coloured  paper.  We  note  that  the  red  paper  still  looks 
red  in  the  red  light,  but  that  the  green  and  blue  paper  or 
ribbons  look  almost  perfectly  black.  On  the  other  hand, 
when  plunged  into  the  green  light,  the  red  paper  or  ribbons 
now  present  an  almost  perfectly  black  appearance,  whilst 
the  green  and  greenish  blue  retain  their  normal  colour. 
In  order  to  understand  in  what  sense  sources  of  light  can 
be  compared  together,  it  is  necessary  to  keep  in  view  the 
fact  that  every  ray  of  light  has  three  qualities  :  firstly,  what 
we  call  its  colour  ;  secondly,  its  luminosity  or  brightness ; 
and  thirdly,  its  purity,  or  the  degree  to  which  that  ray  is 
mixed  with  white  or  other  light. 

When  white  light,  the  standard  of  comparison  being  day- 
light, falls  upon  a  coloured  surface,  be  it  leaf  or  flower,  some 


ELECTRIC  MEASUREMENT.  33 

part  of  the  light  is  reflected  back  unaltered  in  quality.  Some 
portion  passes  through  the  surface,  and  is  reflected  back  by 
internal  reflection,  and  in  performing  the  double  journey 
through  the  surface  layer  it  has  certain  of  its  constituent  rays 
weakened  or  destroyed.  The  apparent  hue  or  colour  of  the 
object  depends  upon  the  nature  of  this  selective  absorption. 
The  brightness  or  luminosity  of  the  surface  depends  upon  the 
intensity  of  the  light  which  falls  upon  it,  and  upon  the 
amount  by  which  that  light  which  is  reflected  from  it,  whether 
superficially  or  internally,  is  weakened.  The  eye  is  sensitive 
to  both  these  qualities  of  colour  and  brightness,  and  can 
pronounce  judgment  upon  either  of  them,  or  on  both.  More- 
over, we  appreciate  the  extent  to  which  the  altered  light  is 
mixed  with  the  unaltered  light.  In  the  case  of  coloured  surfaces 
which  we  call  pale  or  light  in  tint,  such  as  a  light  pink,  or 
light  or  Cambridge  blue,  there  is  a  considerable  admixture  of 
white  light  when  these  surfaces  are  seen  by  normal  daylight. 
Hence,  when  white  light  falls  upon  a  surface,  some  of  it  is 
reflected  unaltered,  and  some  part  is  returned  to  the  eye  after 
having  suffered  a  weakening  or  destruction  of  some,  or  all,  of 
the  rays  present  in  it.  A  surface  we  call  white  reflects  a  large 
fraction  of  the  light  which  falls  upon  it  unaltered  in  quality  ; 
a  surface  we  call  black  reflects  a  very  small  fraction,  mostly 
unaltered,  of  the  light  which  falls  upon  it ;  and  a  surface 
we  call  coloured  sends  back  to  our  eyes  a  fraction  of  the  light 
which  falls  upon  it,  but  the  quality  of  the  light  is  altered  by 
the  diminution  in  brightness  of  some,  or  all,  of  the  various 
rays  present  in  it. 

Suppose  that  there  are  two  white  surfaces,  on  each  of 
which  pure  red  light  from  different  sources  is  being  thrown. 
They  may  differ  in  brightness  or  luminosity,  but  there  is 
generally  no  difficulty  in  deciding  which  of  these  two  surfaces 
is  the  brighter.  If,  however,  we  have  two  surfaces,  one  of 
which  is  reflecting  pure  blue  light  and  the  other  pure  red 


34      ELEGTRIG  LAMPS  AND  ELECTRIC  LIGHTING. 

light,  then  we  have  two  differences  to  appreciate,  namely,  a 
real  difference  in  colour  due  to  the  difference  in  the  wave 
lengths  of  the  lights  reflected  by  these  surfaces,  and  a  differ- 
ence in  brightness  or  luminosity,  which  may,  or  may  not, 
exist,  due  to  the  different  intensities  of  the  lights  reflected 
from  the  two  surfaces.  A  certain  training  of  the  eye  is 
necessary  in  order  to  distinguish  a  difference  in  the  luminosity 
of  two  surfaces,  altogether  apart  from  the  question  whether 
they  have,  or  have  not,  a  difference  of  tint.  An  inexperienced 
eye  cannot  do  this  with  the  exactitude  of  a  trained  eye.  It  is 
comparatively  easy  in  extreme  cases.  If,  for  instance,  I  place 
before  you,  in  a  white  light,  a  piece  of  dark  blue  paper  and  a 
piece  of  light  yellow  paper,  you  have  no  difficulty,  quite  apart 
from  the  colour  difference,  in  deciding  at  once  that  the  blue 
is  darker  than  the  yellow,  or  less  bright  ;  and  if,  on  the  other 
hand,  I  present  to  you  similar  pieces  of  a  light  Cambridge  blue 
and  a  very  dark  red,  you  have  also  no  difficulty  in  deciding  as 
to  the  relative  brightness  of  those  surfaces.  The  artistic  eye 
is  especially  trained  in  this  sort  of  discrimination  of  bright- 
ness or  luminosity,  as  distinguished  from  colour.  An  artist, 
for  instance,  does  not  admit  that  any  objects  are  correctly 
depicted  in  which  the  proper  differences  of  luminosity,  or 
brightness,  or  of  light  and  shade,  as  he  would  say,  are  not 
properly  expressed  or  suggested.  Very  often  it  is  quite  out 
of  the  power  of  the  artist  to  give  the  different  parts  of  the 
surface  of  his  paper  or  canvas  the  same  actual  relative 
luminosity  or  brightness  as  exists  in  the  case  of  the  objects  he 
is  depicting.  He  cannot,  for  instance,  depict  on  his  canvas 
dark  green  leaves  when  seen  against  a  bright  background  of 
white  cloud  or  snow  in  a  manner  which  shall  give  them  the 
proper  relative  brightness  or  luminosity  as  seen  in  Nature, 
especially  when  that  paper  or  canvas  has  to  be  viewed  in  the 
interior  of  a  room.  A  great  part  of  the  art  of  painting 
consists  in  suggesting  differences  of  luminosity  which  cannot 
be  exactly  obtained.  But  when  an  artist  makes  a  sketch  with 


monochrome,  or  crayon,  or  pencil,  he  endeavours  in  some 
sense  to  so  alter  the  surface  of  the  paper  in  its  various  parts 
that,  without  regard  to  colour  differences,  these  patches  of  the 
paper  imitate  more  or  less  nearly  the  relative  luminosity  or 
brightness  of  the  various  parts  of  the  surfaces  of  the  objects 
to  be  depicted. 

When,  therefore,  objects  having  what  we  call  colour  are 
viewed  by  the  aid  of  two  different  illuminants,  each  sending 
out  light  of  different  qualities,  as  far  as  regards  the  colour- 
distinguishing  powers  of  those  two  lights  there  is  no  sense  in 
which  they  can  be  compared.  As  regards  the  power  of  dis- 
tinguishing colours,  we  cannot  compare  an  electric  arc  lamp 
with  a  candle,  because  no  number  of  candles,  however  great, 
are,  or  can  be,  the  equivalent  of  any  arc  lamp  in  their  power 
of  revealing  or  producing  colour  differences.  We  can,  how- 
ever, within  certain  limits,  compare  lights  in  regard  to  their 
different  powers  of  producing  on  a  white  surface  equal 
luminosity  or  brightness,  whether  these  lights  be  monochro- 
matic— that  is  to  say,  emit  rays  of  only  one  wave  length — 
or  whether  they  are  emitting  a  compound  light  composed  of 
rays  of  many  wave  lengths.  Taking  any  surface,  such  as 
white  paper,  which  is  capable  of  reflecting  rays  of  all  wave 
lengths,  at  least  as  far  as  those  which  affect  the  eye  are  con- 
cerned, we  may  illuminate  part  of  this  paper  from  one  source 
of  light  and  part  from  another  source,  and  we  may  adjust  the 
intensity  of  these  two  sources  of  light  in  such  a  way  that  a 
discriminating  eye  can  assert  that  the  two  parts  of  the  white 
surface  are  of  equal  brightness,  whether  they  are  apparently 
of  the  same  colour  or  not.  There  are,  however,  certain  great 
difficulties  in  doing  this  which  must  not  be  ignored  when 
comparing  together  the  illumination  produced  by  two  lights 
of  different  character. 

It  is  necessary  to  bear  in  mind  that  in  making  these 
photometric  comparisons,  our  eye  is  the  only  instrument 

D2 


36      tiLECTBIC  LAMPS  AND  ELECTRIC  LIGHTING. 

which  we  can  employ,  and  we  are  limited  in  our  opera- 
tions by  the  physiological  properties  of  that  organ.  The 
eye  is  wonderfully  susceptible  of  education,  but  there  are 
certain  inherent  properties  of  it  which  no  education  can 
overcome  or  displace.  It  is  often  taken  for  granted  that 
the  physiological  effect — namely,  the  amount  of  visual 
sensation  produced  by  a  given  light — is  proportional  to  the 
luminous  or  intrinsic  brilliancy  of  that  light,  but  this  is  not 
really  the  case.  If  two  white  surfaces  are  illuminated  respec- 
tively by  pure  red  and  pure  blue  light,  and  the  lights  are  ad- 
justed so  that  the  surfaces  are  of  apparently  equal  brilliancy, 
then,  if  both  the  sources  of  light  are  doubled  or  trebled  in 
intensity,  the  relative  illuminations  on  those  surfaces  will  no 
longer  be  equal.  In  other  words,  the  visual  sensation  does 
not  increase  proportionately  to  the  luminous  intensity  of  the 
source  of  light.  The  visual  impression  produced  by  violet 
or  blue  light  increases  more  slowly  than  that  of  red  light 
when  the  absolute  or  intrinsic  intensities  of  the  two  lights 
are  steadily  increased  by  equal  steps.  As  long,  however,  as 
we  are  dealing  with  lights  of  fairly  similar  character,  we  have 
a  practical  basis  for  comparison  in  the  experimental  fact  that 
the  illumination  produced  on  a  white  surface,  due  to  any  such 
source  of  light,  say  a  standard  lamp,  placed  at  any  distance 
from  the  surface,  can  be  exactly  imitated  by  placing  four  such 
equal  sources  of  light  at  double  the  distance  from  the  surface  ; 
provided  that  in  both  cases  the  rays  of  light  fall  in  a  similar 
manner  on  the  surface. 

It  is  owing  to  the  above-mentioned  different  physiological 
action  of  different  coloured  lights  (a  phenomenon  discovered 
by  Purkinje)  that  high  authorities  such  as  Helmholtz 
("  Physiological  Optics,"  p.  420)  have  stated  that  any  com- 
parison of  lights  of  different  colours  is  impossible.  One  fact  is 
perfectly  certain,  and  that  is,  that  there  is  no  way  in  which  we 
can  by  any  simple  number,  such  as  the  candle-power,  express 


ELECTEIC  MEASUREMENT.  37 

the  relative  colour-distinguishing  powers  of  different  lights, 
and  any  attempt  to  do  so  is  pure  nonsense.  Taking  daylight 
from  a  bright  northern  sky  as  our  standard  of  normal  light,  we 
cannot  express  the  degree  in  which  the  light  from  an  electric 
arc  or  glow  lamp  can  reveal  colour  differences  as  seen  by  such 
normal  daylight  by  stating  the  candle-power  of  those  lights, 
and  hence  all  such  expressions  as  that  an  electric  arc  lamp 
has  so  many  candle-power  are  insufficient  and  inaccurate 
methods  of  assigning  a  visual  value  to  the  light.  We  can, 
however,  compare  within  certain  limits  the  relative  brightness 
of  white  surfaces  when  exposed  to  the  two  lights  which  are 
to  be  compared.  For  many  purposes  for  which  we  require 
light,  such  as  reading  or  writing,  this  gives  us  a  measure  of 
the  value  of  the  light  for  visual  purposes.  At  present,  the  only 
scientific  basis  for  photometry  is  to  be  found  in  the  approxi- 
mately correct  fact  that  brightnesses  can  be  added  together,  and 
that  the  illumination  or  brightness  produced  on  a  white  surface 
by  two  separate  lights  is  the  sum  of  the  brightness  produced 
by  the  individual  illuminations.  The  limitations  set  upon 
photometry  by  the  physiological  properties  of  the  human  eye 
seem,  in  some  aspects,  to  be  insurmountable ;  and,  since 
seeing  means  discriminating  colour  and  brightness  difference, 
no  substitute  for  the  eye  which  is  not  an  eye  can  be  found. 
Our  chief  resource  is  the  power  we  have  of  training  the  eye 
to  increased  visual  sensibility,  but  a  person  with  a  naturally 
"  bad  eye  "  for  colour  or  luminosity  difference  will  never  make 
a  good  photometrist. 

In  order  to  effect  such  a  comparison  of  lights  we  must  start, 
first,  with  a  standard  illuminant,  and,  second,  with  a  standard 
of  illumination.  Unfortunately,  the  legal  standard  illuminant 
is  agreed  on  all  hands  to  be  an  exceedingly  unsatisfactory 
standard.  This  standard  is,  by  the  Metropolis  Gas  Act  of 
1860,  defined  to  be  a  standard  sperm  candle  £in.  in  diameter, 
burning  120  grains  in  the  hour,  and  it  is  called  a  standard  or 


38      ELECTEIG  LAMPS  AND  ELECTRIC  LIGHTING. 

parliamentary  candle.  We  will  return  in  a  moment  to  con- 
sider the  various  other  and  much  hetter  standards  of  illumi- 
nation which  have  been  suggested,  but,  for  the  present,  let 
us  assume  that  by  the  candle  we  mean  a  normal  standard 
candle,  or,  at  any  rate,  the  mean  of  a  large  number  of  such 
standard  candles  in  illuminating  power.  The  illumination 
which  such  a  candle  produces  on  a  white  surface,  say  a  sheet 
of  paper,  held  at  a  distance  of  one  foot  from  it,  is  called  one 
candle-foot,  and  the  candle-foot  is  the  unit  of  illumination, 
just  as  the  candle  is  the  unit  of  illuminating  power.  The 
principle  on  which  practical  photometry  is  based  is  the  experi- 
mental fact  that  the  brightness  of  the  surface  of  white  paper 
produced  by  two  candles  at  a  distance  of  one  foot  from  the 
surface  is  twice  that  produced  by  one  candle  at  a  distance  of 
one  foot,  and,  moreover,  that  the  illumination  of  such  a  sur- 
face produced  by  four  candles  at  a  distance  of  two  feet  is 
equal  to  the  illumination  produced  by  one  candle  at  a  distance 
of  one  foot.  Similarly,  nine  candles  at  three  feet  and  six- 
teen candles  at  four  feet  distance  from  the  white  surface 
produce  the  same  illumination  as  does  one  candle  at  a  distance 
of  one  foot.  The  candle-foot  is  found  to  be  an  exceedingly 
convenient  illumination  for  the  purposes  of  reading.  Most 
persons  can  hardly  read  comfortably  if  the  illumination  on 
their  book  or  paper  is  less  than  one  candle-foot.  The  illumina- 
tion in  a  well-lighted  room  may  vary  from  two  candle-feet  on 
the  tables  to  half  a  candle-foot  on  the  walls  and  a  quarter  of 
a  candle-foot  on  the  floor,  while  a  brilliant  illumination,  such 
as  that  required  on  a  theatre  stage,  is  from  three  to  four 
candle-feet.  The  illumination  in  a  street  due  to  the  street 
gas  lamps  may  be  from  one-third  to  one-fourth  of  a  candle- 
foot  ;  in  a  picture  gallery,  from  one  to  three  candle -feet. 
The  illumination  due  to  full,  high  moonlight  in  London  is  only 
g-^th  to  TJffth  of  a  candle-foot,  depending  on  the  clearness  of 
the  sky  and  altitude  of  the  moon.  These  fractional  figures 
must  be  understood  as  follows  : — An  illumination  of  a  quarter 


ELECTRIC  MEASUREMENT.  39 

of  a  candle-foot  means  illumination  which  would  be  produced 
by  a  light  of  one  candle  held  at  two  feet  distance  from  the 
white  surface,  and  an  illumination  of  yjy^-th  of  a  candle-foot  is 
the  illumination  produced  by  a  candle  held  at  a  distance  of 
ten  feet  from  the  white  surface.  The  illumination  of  full  sun- 
light may  be  from  7,000  to  10,000  candle-feet  when  the  sun 
is  high  in  a  bright  clear  climate.  The  mean  daylight  in  the 
interior  of  a  well-lighted  room  may  be  from  ten  to  forty  candle- 
feet.  Attempts  have  been  made  at  various  times  to  estimate 
the  relative  brightness  of  a  white  surface  when  held  in  full 
sunlight  and  when  held  in  full  moonlight.  Various  numbers 
have  been  obtained  by  different  observers.  Bouguer,  in  1725, 
and  Bond,  in  America,  in  1851,  made  estimates  of  the  relative 
brightness  of  sunlight  and  moonlight.  Unless  the  altitudes 
of  the  sun  and  moon  and  atmospheric  conditions  are  stated, 
these  relative  values  do  not  mean  very  much.  Roughly 
speaking,  full  sunlight  produces  an  illumination  from  400,000 
to  700,000  times  as  great  as  that  of  full  moonlight.  It  is 
difficult  to  read  anything  but  fairly  large  type  in  full  moon- 
light.* 


*  Bouguer  dispersed  the  rays  of  the  sun  falling  on  a  small  hole  by  a 
concave  lens  and  compared  the  diffused  sunlight  with  the  light  of  a  candle. 
Bond  formed  the  image  of  the  sun  on  a  sphere  of  silvered  glass,  and  com- 
pared it  when  so  weakened  by  reflection  with  a  standard  light.  Bouguer 
found  that  the  intensity  of  full  sunlight  was  300,000  times  greater  than 
that  of  full  moonlight,  and  Bond  found  the  ratio  to  be  470,980  to  1. 
More  recently  Prof.  Young,  in  America,  has  made  an  estimate  of  the  sun's 
mean  candle-power.  After  correcting  for  atmospheric  absorption,  he  finds 
the  candle-power  of  the  sun  to  be  1,575  x  1024.  Prof.  L.  Weber,  of 
Breslau,  finds  that  the  red  rays  in  sunlight  are  1,110  x  1024  times  more 
intense  than  the  corresponding  rays  in  caudle  light,  and  the  green  rays 
in  sunlight  2,294  x  1024  times  as  intense  as  green  candle-light  rays.  Since 
the  sun's  mean  distance  from  the  earth  is  48  x  1010  feet,  the  sun's  illumina- 

tion at  the  earth  is      '57   x  1Q       =  7,000  candle  feet.     Average  moonlight 


is  7*5  th  of  a  candle-foot,  and  hence  full  sunlight  illumination,  on  a  white 
surface,  is  to  full  moonlight  in  the  ratio  of  490,000  to  1.  (1024  stands  for 
a  billion  times  a  billion.) 


40      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


We  turn,  then,  to  consider  some  of  the  methods  of  photo- 
metric comparison  which  depend  on  the  principle  that  one 
source  of  light  is  said  to  have  an  illuminating  power  equal 
to  that  of  another  when  the  brightness  of  a  white  surface  held 
at  the  same  distance  from  both  these  sources  is  the  same. 
Taking,  for  instance,  the  normal  candle  as  a  standard,  any 
other  source  of  light  is  said  to  have  four  candle-power  if  it 
produces  the  same  apparent  brightness  on  a  white  surface 
held  two  feet  from  it  as  does  a  candle  if  held  one  foot  from 
the  same  surface ;  such  comparison  being  made  wholly  with 
regard  to  luminosity  or  brightness  of  the  white  surface,  and 
no  attention  being  given  to  the  difference  in  colour  in  any 


FIG.  10. — Comparison  of  Illuminating  Power  of  Glow  Lamp  and  Candle 
by  means  of  Ritchie's  Wedge. 

white  surface  when  viewed  by  the  two  sources  of  light.  The 
simplest  method  of  effecting  this  comparison  is  by  the  use  of 
Ritchie's  wedge.  A  block  of  wood  is  cut  in  the  shape  of  a 
wedge  with  a  rather  obtuse  angle,  and  has  its  two  adjacent 
surfaces  covered  with  fine  white  writing  paper,  the  separating 
edge  being  made  as  sharp  as  possible  (see  Fig.  10).  The  two 
sources  of  light  which  are  to  be  compared — say  a  candle  and 
an  incandescent  lamp — a.re  placed  together  in  a  darkened 
room.  The  wedge  is  held  between  them,  with  its  edge 
vertical,  and  on  looking  at  the  vertical  edge  we  see  that  one 
side  of  the  wedge  is  illuminated  by  the  candle  and  the  other 
surface  by  the  electric  glow  lamp.  We  can  then  move  the 


ELECTRIC  MEASUREMENT.  41 

wedge  to  and  fro  until  we  find  a  position  such  that  the  two 
sides  of  the  wedge  respectively  illuminated  by  the  two  agents 
appear  to  be  of  the  same  brightness.  We  are  assisted  in 
getting  rid  of  any  difficulty  due  to  difference  of  colour  in  the 
light  by  adopting  the  principle  suggested  by  Capt.  Abney, 
viz.,  by  oscillating  the  wedge  or  swinging  it  to  and  fro. 
When  we  have  very  nearly  found  the  position  of  balance,  if 
we  move  the  wedge  to  and  fro  in  slowly  diminishing  arcs,  we 
shall  alternately  increase  and  diminish  the  relative  brightness 
of  the  two  surfaces,  but  we  shall  not  affect  any  difference  in  their 
relative  colour  tints  ;  hence  the  attention  of  the  eye  is  by  this 
device  fastened  upon  that  which  is  varying,  namely,  the  relative 
luminosity  or  brightness  of  the  two  surfaces,  and  distracted 
from  that  to  which  we  wish  to  give  no  attention,  namely, 
their  relative  colour  difference.  Having  found  the  position  in 
which  the  two  inclined  surfaces  of  the  wedge  appear  equally 
bright  when  each  respectively  is  illuminated  by  one  of  the  two 
sources,  we  measure  the  distance  from  the  candle  to  the 
wedge  and  from  the  lamp  to  the  wedge.  The  illumination  pro- 
duced by  the  candle  is  then  obtained  by  dividing  unity,  since 
the  candle  is  the  unit  of  illuminating  power,  by  the  square  of 
its  distance  in  feet  from  the  wedge ;  and,  likewise,  the  illu- 
mination produced  by  the  electric  glow  lamp  is  equal  to  its 
unknown  candle-power  divided  by  the  square  of  its  distance 
in  feet  from  the  wedge.  Since  these  illuminations  are  equal, 
it  follows  that  the  candle-power  of  the  glow  lamp  must  be 
numerically  given  by  dividing  the  square  of  the  distance  of 
the  glow  lamp  from  the  wedge  by  the  square  of  the  distance 
of  the  candle  from  the  wedge.  Thus,  if  the  balance  has  been 
found  when  the  glow  lamp  is  four  feet  from  the  wedge  and 
the  candle  is  one  foot  from  the  wedge,  the  candle-power  of  the 
glow  lamp  is  four  times  four  divided  by  one  times  one,  or  16. 

Instead  of  employing  a  wedge,  some  observers  make  use  of 
a  method  due  to  Ilumford,  in  which  the  two  illuminants  are 


42      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


made  to  cast  the  shadow  of  a  stick  upon  a  white  screen. 
If  a  glow  lamp  and  a  candle  are  placed  a  short  distance 
apart  (see  Fig.  11),  and  a  pencil  or  other  rod  of  wood  is 
held  an  inch  or  two  from  a  white  card,  which  is  placed 
so  as  to  be  equally  illuminated  by  the  two  sources  of  light, 
it  will  be  found  that  there  are  two  shadows  on  the  card. 
One  is  a  shadow  due  to  the  lamp,  and  the  other  is  a  shadow 
due  to  the  candle.  The  shadow  due  to  the  candle  is  a 
space  into  which  candle-light  does  not  shine,  and  upon 
which  the  light  from  the  lamp  shines,  whilst  the  shadow 
due  to  the  lamp  is  a  portion  of  the  surface  which  is 
illuminated  by  candle-light  alone.  By  suitably  moving  the 


FIG.  11. — Comparison  of  Illuminating  Power  of  Glow  Lamp  and  Candle 
by  means  of  Kumford  Shadow  Method. 


candle  and  the  lamp  we  find  we  can  obtain  positions  in 
which  the  two  shadows  are  of  equal  depth,  and  we  then 
obtain  the  candle-power  of  the  incandescent  lamp  by  dividing 
the  square  of  its  distance  in  feet  from  the  screen  by 
the  square  of  the  distance  in  feet  of  the  candle  from  the 
screen.  All  that  we  are  doing  in  this  case  is  to  compare 
together  the  illumination  produced  upon  a  white  surface 
which  is  partly  illuminated  from  one  source  and  partly  from 
another,  and  the  advantage  which  the  rod  gives  us  is  that 
we  can  make  these  two  shadows  or  illuminated  surfaces  just 
touch  one  another,  and  so  be  assisted  in  making  a  comparison 
between  their  relative  brightnesses.  If  the  same  experiment  is 


ELECTRIC  MEASUREMENT.  43 

tried  with  moonlight  and  a  candle,  the  observer  will  very  soon 
realise  the  difficulties  that  attend  photometric  measurement 
when  the  lights  are  of  different  qualities.  On  any  bright 
moonlight  night  hold  a  white  piece  of  card  at  the  window, 
and  place  a  candle  so  as  to  illuminate  the  card  at  about  the 
same  angle.  Hold  a  pencil  in  front  of  the  card,  and  it  will 
be  seen  that  there  are  two  shadows,  one  of  which  is  a  bright 
blue  and  the  other  a  bright  yellow.  The  yellow  shadow  is, 
apparently,  the  shadow  thrown  by  the  moon,  and  the  blue 
shadow  is,  apparently,  the  shadow  thrown  by  the  candle.  In 
reality,  that  which  we  take  for  the  shadow  due  to  the  moon  is 
a  space  illuminated  by  candle-light  alone,  and  the  shadow  we 
take  for  the  shadow  produced  by  the  candle  is  a  space  on  the 
card  illuminated  by  moonlight  alone.*  These  two  surfaces 
differ  not  only  in  colour,  but  in  brightness,  because  the  light 
falling  upon  them  is  very  different  in  quality.  By  moving 
the  candle  we  can  find  a  position,  generally  from  seven  to  ten 
feet  from  the  card,  in  which  the  blue  and  yellow  shadows, 
though  different  in  colour,  have  apparently  the  same  depth. 
But  the  difficulty  of  determining  when  this  relative  luminosity 
is  the  same  will  give  an  observer  who  tries  the  experiment 
for  the  first  time  an  insight  into  the  difficulties  of  photometric 
measurement. 

A  third  and  more  frequently-used  method  for  the  com- 
parison of  sources  of  illumination  is  the  grease-spot  photo- 
meter of  Bunsen.  If  a  piece  of  thin  paper  has  a  spot  of 
grease  placed  upon  it,  and  if  we  hold  up  the  paper  between 
our  eye  and  the  light,  we  find  the  grease- spot  more  trans- 
parent than  the  rest  of  the  paper,  and  it  looks  lighter.  But 
if  we  hold  the  paper  so  that  the  light  falls  on  it,  we  see  that 
the  grease-spot  is  apparently  darker  than  the  rest  of  the  paper. 

*  It  was  this  and  other  similar  phenomena  which  drew  Goethe  into 
some  confusion  of  thought  on  the  subject  of  colour,  and  led  him  in  his 
to  dispute  Newton's  theory  of  colour. 


44      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


If  the  disc  of  paper  with  a  grease-spot  upon  it  (see  Fig.  12)  is 
held  up  between  two  lights,  such  as  a  gas  flame  and  a  candle, 
and  looked  at,  first  from  one  side  and  then  from  the  other,  it 
will  be  found  possible,  by  moving  the  disc,  to  find  a  position 
for  the  piece  of  paper  such  that  the  grease-spot  can  hardly  be 
seen  from  either  side.  When  this  is  the  case  the  two  sur- 
faces of  the  paper  are  equally  illuminated  by  the  two  sources 
of  light,  and  if  one  of  these  sources  of  light  is  a  standard 
candle,  then,  as  above  explained,  the  candle-power  of  the 
other  source  of  light  is  obtained  by  dividing  the  square  of  the 
distance  in  feet  of  the  source  of  light  from  the  screen  by  the 
square  of  the  distance  in  feet  of  the  candle  from  the  screen. 


FIG.  12. — Comparison  of  Illuminating  Power  of  Glow  Lamp  and  Candle 
by  means  of  Bunsen  Grease-Spot  Method. 

In  practice  various  devices  are  used,  such  as  a  pair  of  inclined 
mirrors  at  the  back  of  the  disc,  and  arrangements  for  rever- 
sing the  disc,  to  enable  the  observer  to  rapidly  make  a  fair 
comparison  between  the  two  surfaces  of  the  disc,  and  to 
ascertain  if,  when  equally  viewed  from  both  sides,  the  grease- 
spot  is  invisible.  The  grease-spot  method  is  one  which  is 
largely  used  in  the  determination  of  the  candle-power  of 
illuminating  gas.  The  Gas  Companies  are  bound  to  supply 
gas  of  a  certain  candle-power  when  burnt  from  a  particular 
standard  burner,  and  official  comparisons  have  to  be  made  in 
order  to  determine  if  they  comply  with  their  obligations,  If 


ELECTRIC  MEASUREMENT.  45 

the  grease-spot  or  shadow  methods  are  employed  for  the  com- 
parison of  a  candle  with  a  glow  lamp,  we  do  not  find  any 
serious  difficulties  in  determining  the  relative  brightness  of 
the  white  surfaces  illuminated  by  the  two  sources  of  light, 
because  the  qualities  of  these  two  lights  are  not  very  diffe- 
rent— tli  at  is  to  say,  if  we  formed  a  spectrum  from  each  light 
we  should  find  that  the  different  rays  in  these  spectra,  which 
are  of  the  same  wave  length,  can  all  be  made  to  be  equal  in 
luminosity ;  or,  in  other  words,  if  the  luminosity  of  the  red 
ray  in  the  glow  lamp  is  equal  to  the  luminosity  of  the  red 
ray  of  the  same  wave  length  in  the  candle,  then  the  green, 
blue  and  violet  rays  of  corresponding  wave  lengths  are 
also  equal  as  regards  luminosity.  If  the  experiment  is  tried 
with  two  lights  of  very  different  qualities,  such  as 
an  electric  glow  lamp  and  an  electric  arc  lamp,  con- 
siderable difficulty  will  be  found  by  the  unpractised  observer 
in  determining  when  two  adjacent  surfaces  illuminated  by 
these  two  sources  of  light  are  of  equal  brightness.  In  order 
to  diminish  this  difficulty,  observers  have  sometimes  employed 
coloured  glasses  to  select  particular  rays  from  the  arc  lamp 
and  compare  them  with  similar  rays  in  the  candle-light, 
and  numerous  determinations  exist  of  the  candle-power  of 
different  arc  lamps  for  red  and  green  rays.  But  this  really 
serves  no  useful  purpose.  The  practical  value  of  a  light  is  not 
the  relative  intensities  of  certain  rays  when  compared  with 
rays  of  the  same  wave  length  in  the  light  of  a  candle,  but  the 
total  brightness  which  that  light  is  capable  of  producing  on  a 
white  surface  when  compared  with  the  total  brightness  which 
the  unit  illuminant  is  capable  of  producing  on  the  same  sur- 
face. 

We  may  further  illustrate  by  an  experiment  the  diffi- 
culty of  comparing  together  the  brightness  of  two  surfaces  of 
different  colour  in  the  following  manner : — An  electric  glow 
lamp  with  a  red  glass  bulb  and  an  electric  glow  lamp  with  a 


46      ELECTRIC  LAMPS  AND  ELECT&IC  LIGHTING. 

green  glass  bulb  (see  Fig.  13)  are  so  placed  as  to  equally 
illuminate  a  white  surface.  A  rod  or  other  opaque 
body  is  then  placed  so  as  to  cast  two  shadows,  one 
from  each  lamp.  You  observe  that  the  two  shadows 
are  respectively  red  and  green;  the  red  lamp  appears  to 
cast  a  green  shadow  and  the  green  lamp  appears  to  cast 
a  red  shadow,  and  you  will  find  considerably  more  difficulty 
in  comparing  together  these  two  lights  by  this  Bumford 
method  of  photometry  than  would  be  the  case  if  the  two 
lights  were  both  of  them  white  or  both  of  them  red.  By 
adopting  the  method  of  oscillating  one  of  the  lights  so  as  to 


Green  Glass    Red  Q|ass 


Bulb 


Bulb. 


FIG.  13. — Comparison  of  Illuminating  Power  of  Two  Lights  of  Different 
Colours  by  Shadow  Method.  The  two  glow  lamps  have  glass  bulbs  of 
different  colours. 

alternately  increase  and  diminish  the  brightness  of  one  of  the 
shadows,  the  difficulty  of  making  this  luminosity  comparison 
is  partly  overcome. 

With  regard  to  the  standards  of  illuminating  power, 
it  has  been  previously  stated  that  the  British  standard 
candle  is  an  unsatisfactory  standard.  The  French  adopted 
a  standard  oil  lamp,  called  a  carcel  standard  lamp,  burn- 
ing 42  grammes  of  colza  oil  per  hour  with  a  wick  of  a 
certain  size.  This  carcel  standard  gives  a  light  equal  to 


MEASUREMENT.  4? 

about  9J  British  candles.  A  standard  which  has  been  adopted 
to  some  extent  in  Germany,  and  to  a  less  extent  in  England, 
is  the  Hefner- Alteneck  amyl-acetate  lamp.  This  is  a  little 
spirit  lamp  burning  pure  pear  oil  from  a  wick  of  certain 
size,  and  yielding  a  flame  40  millimetres  high,  with  an 
illuminating  power  of  rather  less  than  one  British  candle. 
But  the  objection  to  its  use  as  a  standard  is  that  it  gives  a 
somewhat  reddish  light.  Another  standard  largely  used  in 
England  is  the  Methven  gas  standard.  In  this  standard, 
ordinary  coal  gas,  enriched  with  benzol  or  pentane,  two 
hydro -carbon  liquids,  is  burnt  at  a  particular  form  of 
argand  burner.  In  front  of  the  flame  is  placed  a  metal  plate, 
having  a  slit  in  it  of  such  a  size  that  it  only  permits  light 
to  pass  from  the  centre  of  the  flame,  and  this  slit  is  ad- 
justed so  that  the  light  is  equal  to  two  standard  candles. 
Variations  of  quality  and  pressure  in  the  gas  are  said  not  to 
affect  the  intensity  of  the  light  which  is  emitted  through  the 
slit.  In  practice  this  is  not  found  to  be  exactly  the  case.  By 
far  the  most  satisfactory  standard  which  has  yet  been  pro- 
posed is  the  pentane  air-gas  standard  of  Mr.  Vernon  Harcourt. 
In  this  standard  a  mixture  of  volatile  hydro-carbon  (called 
pentane)  and  air  is  burnt  at  a  jet  of  a  certain  kind  in  such  a 
manner  as  to  yield  a  flame,  produced  by  the  combustion  of  a 
definite  chemical  compound,  such  flame  having  a  certain 
height  and  dimensions.  By  taking  suitable  precautions,  this 
pentane  gas  standard  can  be  made  to  yield  excellent  results 
as  a  standard  of  light.  It  has  been  proposed  by  M.  Violle,  a 
distinguished  French  chemist,  that  an  absolute  standard  of 
light  should  be  obtained  by  taking  the  light  emitted  from  one 
square  centimetre  of  molten  platinum.  Such  a  standard,  as 
a  practical  standard  of  reference,  is  not  very  convenient  or 
suitable  for  use  as  a  general  primary  standard  of  light,  but 
it  may  be  found  valuable  as  an  ultimate  reference.  For  some 
purposes  the  only  satisfactory  method  of  comparing  together 
two  sources  of  light,  and  examining  their  qualities  in  regard 


48      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

to  the  relative  proportions  and  brightness  of  the  different  rays 
existing  in  those  lights,  is  by  employing  the  spectro-photo- 
meter.  This  instrument  consists  of  an  arrangement  whereby 
the  same  prism  is  made  to  analyse  the  light  from  two  sources 
of  light,  and  to  expand  the  rays  sent  out  from  these  two 
sources  of  light  into  two  rainbow  bands  or  spectra  in  such  a 
manner  that  these  spectra  are  placed  one  above  the  other,  the 
corresponding  colours  being  vertically  over  one  another.  It 
is  then  possible  to  weaken  one  of  these  sources  of  light  by 
interposing  either  a  disc  with  an  aperture  in  it  of  variable 
size,  revolving  at  a  rapid  rate,  or  by  employing  a  pair  of  nicol 
prisms,  by  means  of  which  the  light  from  one  source  can  be 
weakened  to  any  extent.  The  brighter  of  these  sources  of 
light  is  weakened  until  one  particular  ray,  say  the  yellow  ray, 
in  that  light  is  exactly  equal  in  brightness  to  the  correspond- 
ing yellow  ray  in  the  spectrum  of  the  other  source  of  light. 
In  other  words,  the  spectra  are  made  to  match  each  other 
exactly  at  this  particular  spot.  It  will  then  be  found  that 
if  the  two  sources  of  light  are  of  different  qualities,  say  a  glow 
lamp  and  an  arc  lamp,  the  two  spectra  will  not  match  each 
other  as  regards  brightness  in  any  other  places. 

Prof.  Nichols,  of  Cornell  University,  U.S.A.,  has  carried 
out  a  series  of  observations  with  an  apparatus  of  this  kind. 
Taking  an  Edison  16-candle-power  incandescent  lamp  as  his 
standard,  he  calls  the  brightness  of  the  spectrum  of  this  light 
everywhere  unity,  and  he  compares  with  it  the  spectrum  of 
any  other  light,  say  an  arc  lamp,  of  which  the  spectrum  has 
been  weakened  so  as  to  make  it  of  identical  brightness  for  one 
particular  yellow  ray  with  the  corresponding  ray  in  the 
spectrum  of  the  glow  lamp.  It  is  then  found  that  the 
spectrum  of  the  arc  lamp  is  less  bright  in  the  red  rays,  but 
much  more  bright  in  the  green,  and  especially  much  more 
bright  in  the  violet.  In  the  spectrum  of  the  arc  lamp, 
especially  when  that  spectrum  is  taken  in  such  a  way  as  to 
utilise  a  large  portion  of  the  light  proceeding  from  the  carbon 


ELECTRIC  MEASUREMENT. 


49 


vapour  between  the  poles,  a  very  bright  violet  band  is  found  at 
the  extreme  end  of  the  spectrum.  These  observations  have 
been  expressed  by  Prof.  Nichols  in  a  series  of  curves  (see 
Fig.  14).  The  horizontal  line  at  the  base  of  the  diagram 
represents  the  spectrums ;  the  letters  A,  B,  D,  E,  &c.,  indicate 


the  position  of  the  characteristic  black  lines  or  missing  rays 
in  the  solar  spectrum,  which  are  taken  as  points  of  reference. 
The  intensity  of  the  spectrum  of  the  glow  lamp  is  taken 
everywhere  as  unity,  and  is  represented  by  the  altitude  of  the 
horizontal  line  marked  glow  lamp.  The  other  curves  then 


50      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

represent,  by  their  ordinates  or  heights  at  various  points,  the 
relative  brightness  of  the  respective  rays  in  the  different 
portions  of  the  spectra  of  the  light  taken  from  the  sun,  the 
arc  lamp;  and  magnesium  wire,  compared  with  the  ray  of 
the  same  wave  length  in  the  light  from  the  electric  glow  lamp. 
It  will  be  Seen  how  enormously  brighter  the  sunlight  and  the 
arc  light  spectrum  are  in  the  neighbourhood  of  the  violet  rays 
when  compared  with  the  corresponding  ray  of  the  glow  lamp, 
but  that  their  light,  relatively  speaking,  is  less  bright  in  the 
red  than  the  corresponding  light  of  the  glow  lamp,  provided 
that  the  spectra  of  all  these  sources  of  light  have  been 
equalised  so  as  to  make  them  of  identical  brightness  in  the 
neighbourhood  of  the  D  line  of  the  solar  spectrum— that 
is,  of  the  yellow  rays. 


LECTURE  II. 


THE  Evolution  of  the  Incandescent  Lamp. — The  Nature  of  the  Problem. — 
Necessary  Conditions. — Allotropic  Forma  of  Carbon. — The  Modern 
Glow  Lamp. — Processes  for  the  Manufacture  of  the  Filament. — Edison- 
Swan  Lamps. — The  Expansion  of  Carbon  when  Heated. — Various 
Forms  of  Glow  Lamps. — Focus  Lamps. — High  Candle-power  Lamps. — 
Velocity  of  Molecules  of  Gases. — Kinetic  Theory  of  Gases. — Processes 
for  the  Production  of  High  Vacua. — Necessity  for  a  Vacuum. — Mean 
free  path  of  Gaseous  Molecules. — Voltage,  Current  and  Candle-power 
of  Lamps. — Watts  per  Candle-power. — Characteristic  Curves  of 
Lamps. — Life  of  Glow  Lamps. — Molecular  Shadows. — Blackening  of 
Glow  Lamps. — Self-recording  Voltmeters. — Necessity  for  Constant 
Pressure  of  Supply. — Changes  Produced  in  Lamps  by  Age.—  Smashing 
Point. — Efficiency  of  Glow  Lamps. — Statistics  of  Age. — Variation  of 
Candle-power  with  Varying  Voltage. — Cost  of  Incandescent  Lighting. 
— Useful  Life  of  Lamps.— Importance  of  Careful "  Wiring."— Average 
Energy  Consumption  of  Lamps  in  Various  Places. — Load  Factors. — 
Methods  of  Glow-Lamp  Illumination  for  Production  of  Best  Effects. 
— Artistic  Electric  Lighting. — Molecular 
Physics  of  the  Glow-Lamp. — High-Voltage 
Lamps. — Varieties  of  Carbon. — Densities 
and  Resistances. — Deterioration  of  Carbon 
Lamps. — High  Efficiency  Lamps. — Recen  tly 
Suggested  Improvements. 


HE  illustrations  and  explanations 
in  the  previous  Lecture  will  have 
prepared  the  way  for  a  study  of  the 
electric  incandescent  lamp  as  a  source 
of  illumination.  We  shall  not  occupy 
time  by  describing  the  stages  by 
which  the  glow  lamp  has  attained 
its  present  perfection.  Neither  is 
it  necessary  to  dwell  upon  questions 
of  scientific  or  legal  priority  which  are  not  most  advantageously 
discussed  from  the  lecture  table.  Suffice  it  to  say  that  in  its 


E2 


52      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

modern  form  the  carbon  filament  electric  lamp  has  been,  and 
always  will  be,  inseparably  associated  with  the  names  of  Edison 
and  Swan,  and,  in  a  lesser  degree,  with  Lane-Fox,  Sawyer  and 
Man.  These  inventors  were  preceded  by  many  who  attempted 
solutions  of  the  problem,  and  they  have  been  followed  by 
others  who  have  added  countless  details  of  invention  to  the 
methods  and  means  by  which  progress  has  been  made  from 
early  ideas  to  a  perfection  which  is  even  now  not  yet  final. 
These  successful  workers,  however,  started  from  a  standpoint 
similar  to  that  which  initiated  the  ideas  and  failures  of  early 
pioneers  in  this  region  of  discovery  and  invention.  The  know- 
ledge of  the  simple  fact  that  a  metallic  wire  or  other  substance 
could  be  heated  by  an  electric  current,  and  that  such  a  heated 
body  when  raised  to  a  proper  temperature  gave  out  rays  of 
light,  was  the  starting  point  for  all  the  attempts  made  by  early 
and  by  later  inventors  to  discover  a  means  of  producing  a 
conductor  which  would  fulfil  the  following  conditions: — (1) 
It  must  be  capable  of  being  heated  to  a  high  temperature 
without  fusion  or  sensible  volatilisation  ;  (2)  it  must  be  a 
material  of  high  specific  resistance ;  and  (3)  it  must  be 
capable  of  being  shaped  into  such  a  form  as  to  be  conveniently 
and  economically  utilised  as  a  means  of  producing  small  units 
of  light. 

In  its  most  widely  used  form  the  carbon  filament  glow  lamp 
is  based  upon  the  fact  that  carbon  can  be  treated  in  a  highly 
perfect  vacuum  or  in  rarefied  gases  to  a  temperature  near,  or 
higher  than,  the  melting  point  of  platinum  without  suffering 
very  rapid  change,  and  that  in  certain  forms  it  fulfils  the  three 
conditions  named  above,  as  necessary  for  the  incandescing 
material  in  a  glow  lamp.  Carbon  is  capable  of  existing  in 
three  modifications,  which  are  chemically  termed  allotropic 
forms.  These  are  :  charcoal  as  ordinarily  obtained  by  the 
carbonisation  of  some  organic  substance,  such  as  wood,  paper, 
thread,  silk,  and  other  similar  substances,  which  consist 


ELECTRIC  GLOW  LAMPS.  53 

principally  of  carbon  united  with  other  elements,  which 
can  be  driven  off  by  heat.  Next,  carbon  exists  in  the  form 
known  as  grapkitt,  and  we  have  it  in  this  form  in  plumbago 
as  used  in  lead  pencils,  and  in  the  hard  carbon  deposit  in  gas 
retorts.  And,  lastly,  carbon  occurs  in  a  transparent  or  opaque 
crystalline  form  known  as  diamond.  All  forms  of  carbon  are 
by  a  sufficiently  high  temperature  converted  into  graphite. 
Of  these  three  forms  of  carbon,  diamond  is  practically  a  non- 
conductor of  electricity  ;  ordinary  charcoal  or  carbon,  such  as 
that  produced  by  carbonising  wood,  cotton,  or  other  organic 
materials,  has  an  electrical  conductivity  which  varies  greatly 
with  the  circumstances  under  which  it  is  produced ;  but,  ap- 
proximately speaking,  it  may  be  said  that  the  hard  and 
dense  form  of  carbon  which  is  obtained  from  gas  retorts  as  a 
product  of  the  distillation  of  coal,  and  which  is  very  largely 
composed  of  graphitic  carbon,  has  an  electrical  resistance 
which  is,  for  the  same  volume,  from  one  to  three  thousand 
times  that  of  copper.  The  temperature  of  fusion  and  tem- 
perature of  volatilisation  of  carbon  is  also  very  high.  If  car- 
bon is  heated  to  a  temperature  near  to  red  heat  in  ordinary 
air,  or  in  an  atmosphere  containing  oxygen,  an  immediate 
combustion  of  the  carbon  takes  place,  produced  by  the  union 
of  the  carbon  with  the  oxygen  of  the  surrounding  atmosphere 
and  the  resulting  production  of  carbonic  acid  gas.  Hence,  in 
order  that  permanence  may  be  obtained,  it  is  essential  that  the 
heating  of  the  carbon  should  take  place  either  in  a  vacuous 
space  or  else  in  an  atmosphere  which  does  not  contain  free 
oxygen.  The  practical  invention  of  the  incandescent  lamp, 
therefore,  turned  upon  the  discovery  of  the  proper  method  for 
producing  carbon  in  the  form  of  a  thin  wire,  thread,  or 
filament,  as  it  is  called,  and  supporting  this  carbon  filament 
in  a  glass  bulb  quite  free  from  all  oxidising  gases  or  air. 

The  final  outcome  of  failure  and  research  was  ultimately 
to  show  that  in  order  to  obtain  light  by  the  incandescence  of 


54     ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

carbon,  such  incandescence  being  produced  electrically,  the 
following  conditions  must  be  met: — First,  the  carbon 
must  bs  in  the  form  of  a  flexible  strip,  wire  or  thread, 
technically  termed  a  "filament,"  and  must  be  in  a  dense 
conductive  form  produced  by  heating  some  organic  substance 
to  a  very  high  temperature.  An  additional  deposit  of 
carbon  is  generally  put  upon  the  filament  by  a  process  called 
"treating."  Secondly,  the  carbon  must  be  enclosed  in  a  bulb 
which  is  exhausted  of  its  air  as  perfectly  as  possible.  It  has 
been  frequently  asserted  by  different  inventors  that  carbon  is 
equally  or  more  permanent  when  contained  in  glass  bulbs 
filled  with  rarefied  gases,  such  as  nitrogen  or  hydrogen, 
chlorine,  bromine,  or  hydrocarbon  gases  ;  but  it  has  not  yet 
been  definitely  shown  that  these  methods  are  as  efficient  as 
the  employment  of  a  very  high  vacuum.  Thirdly,  the  enclos- 
ing bulb  must  be  entirely  of  glass.  Fourthly,  the  electric 
current  must  be  got  into  and  out  of  the  conductor  by  means 
of  platinum  wires  sealed  through  the  glass.  Without  stop- 
ping to  detail  the  attempts  which  have  been  made,  or  which 
are  still  being  made  to  modify  these  conditions,  or  to  con- 
struct lamps  to  give  light  by  incandescenca  which  do  not 
fulfil  them,  suffice  it  to  say  that  the  electric  glow  lamps  in 
general  use  (in  1899)  all  comprise  the  above  four  elements : 
carbon,  vacuum,  platinum,  glass.  Let  us  consider  each  of 
them  in  turn. 

The  carbon  conductor  or  filament  is  made  by  carbonising 
some  material  which  must  be  capable  of  being  reduced  to 
nearly  pure  carbon  after  it  has  been  given  the  necessary 
filamentary  form.  This  carbonisation  is  conducted  by  heat- 
ing a  suitable  material  reduced  to  a  thread-like  form  to  a  very 
high  temperature  in  a  closed  crucible.  If  a  piece  of  paper, 
linen,  cotton  or  thread,  which  consists  essentially  of  cellulose 
(the  chemical  constitution  of  which  is  carbon  united  to  oxygen 
and  hydrogen),  is  raised  to  a  high  temperature,  say  a  white 


ELECTRIC  GLOW  LAMPS.  55 

heat,  in  a  closed  crucible  made  of  very  infusible  material, 
such  as  compressed  blacklead  or  plumbago,  the  hydrogen 
and  oxygen  are  driven  out  from  their  combination  with 
the  carbon,  and  a  residue  is  obtained  which  is  more  or 
less  pure  carbon.  The  temperature  at  which  this  carbonisa- 
tion is  conducted  has  a  great  effect  upon  the  kind  of  carbon 
produced.  Carbonisation  at  a  very  high  temperature,  near  the 
melting  point  of  steel,  when  conducted  out  of  contact  with  air, 
produces  a  highly  dense  and  elastic  form  of  carbon.  Some- 
times the  material  employed  for  this  purpose  is  cotton  thread 
of  a  pure  kind  which  has  been  parckmentised  by  being  treated 
in  a  bath  of  sulphuric  acid,  commonly  known  as  oil  of  vitriol, 
and  water.  This  bath  destroys  the  fibrous  structure  of  the 
thread,  and  produces  a  material  somewhat  resembling  catgut. 
Parchmentised  paper  is  not  infrequently  used  as  a  material  for 
the  covers  of  jam-pots. 

The  dilute  sulphuric  acid  with  which  the  thread  is  treated 
has  the  power  of  producing  a  change  in  the  cellulose,  which 
results  in  the  formation  of  the  above  tough  material.  The 
parchmentised  cotton  thread  is  cut  into  the  necessary  lengths, 
wound  on  a  frame  made  of  carbon  rods,  and  buried  in  a 
crucible  which  is  packed  full  of  powdered  plumbago.  The 
closed  crucible  is  then  exposed  to  a  vivid  white  heat  in  a 
furnace,  and  after  a  sufficient  exposure  to  this  temperature 
the  crucible  is  opened  and  the  carbonised  thread  is  taken  out. 
It  is  then  found  that  the  threads  have  been  converted  into  a 
material  which  is  almost  wholly  pure  carbon  of  a  dense  variety 
and  highly  elastic.  In  place  of  using  parchmentised  thread, 
as  first  suggested  by  Mr.  J.  W.  Swan,  other  means  have  been 
adopted  for  preparing  the  cellulose  in  the  form  of  a  fine 
structureless  thread.  Pure  cotton  wool,  which  is  nearly  pure 
cellulose,  is  capable  of  being  dissolved  by  several  chemical 
materials,  such  as  chloride  of  zinc,  and  the  gummy  mass  so 
produced  can  be  pressed  out  into  a  thread.  This  thread  is 


56      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

then  treated  as  above  described,  and  converted  into  carbonised 
thread  or  carbon  filaments  of  the  necessary  form.  When  so 
prepared  the  carbon  thread  is  an  elastic  material,  having  an 
electrical  resistance  which  is  approximately  two  thousand 
five  hundred  times  that  of  a  copper  wire  of  the  same 
length  and  thickness.  An  immense  variety  of  materials  have 
been  used  and  suggested  for  producing  the  carbon  filament. 
At  one  time  Edison  used  largely  very  thin  slips  of  Japanese 
bamboo  cut  into  the  requisite  shape.  We  will  project  upon  the 
screen  the  image  of  an  Edison  lamp,  of  which  the  carbon  fila- 
ment is  formed  of  carbonised  bamboo  and  has  been  broken  on 


FIG.  15. — Modern  Electric  Glow  Lamp  (Edison-Swan),  with  Looped 
Carbon  Filament. 


one  side,  and  set  it  in  vibration;  you  will  see  by  the  rapid 
movement  of  the  carbon  loop  when  the  lamp  is  shaken 
how  exceedingly  elastic  is  the  delicate  carbon  filament. 
Spiral  springs  can  even  be  prepared  in  this  manner  from  car- 
bonised thread  which  have  a  considerable  degree  of  elasticity 
and  tenacity. 

The  carbon  thread,  having  been  thus  prepared,  has  next 
to  be  mounted  in  a  glass  bulb,  and  it  must  be  so  held  that 


ELECTRIC  GLOW  LAMPS.  57 

when  heated  the  expansion  which  it  experiences  can  be 
permitted  to  take  place  without  endangering  its  rupture 
or  severance  from  the  wires  to  which  its  ends  are  fastened. 
Early  experimentalists  found  an  insuperable  difficulty  in 
connecting  their  carbons  to  the  leading-in  wires,  and  in 
constructing  any  practical  lamp  by  the  use  of  straight 
carbon  rods.  In  most  modern  lamps  the  carbon  conductor 
is  given  a  horse-shoe  or  double-loop  form,  as  shown  in  Fig.  15, 
in  order  that  it  may  have  perfect  liberty  to  expand  when 
heated  without  becoming  detached  from,  or  from  putting  any 
strain  upon,  the  supporting  wires  by  which  it  is  held.  It  is 
very  easy  to  show  that  carbon,  like  most  other  solid  bodies, 
expands  when  heated.  I  have  here  an  Edison-Swan  incandes- 
cent lamp  with  a  long  straight  carbon  filament  (see  Fig.  16). 
One  end  of  the  carbon  filament  is  attached  to  a  platinum  wire 


FIG.  16.— Electric  Glow  Lamp  with  Straight  Carbon  Filament  for  showing 
the  Expansion  of  the  Carbon. 


sealed  through  the  glass  at  the  top,  and  the  other  end  is 
attached  to  a  platinum  wire  sealed  through  the  other  end  of 
the  glass,  but  a  little  spiral  spring  of  steel  is  inserted  between 
the  leading-in  wire  and  the  carbon.  Projecting  an  image  of 
this  upon  the  screen,  and  asking  you  to  fix  your  attention 
upon  the  end  of  the  carbon  attached  to  the  spiral  spring,  it 
will  be  seen  that  when  the  electric  current  is  sent  through  the 
carbon  filament,  making  it  incandescent,  the  spring  contracts, 
thus  showing  the  expansion  of  the  carbon  thread.  If  the 
carbon  is  rigid  and  is  not  given  a  loop  or  horse-shoe  form, 
then  it  is  necessary  to  make  provision  for  this  expansion  of 
the  carbon  by  attaching  it  to  flexible  supports  or  springs,  as 
was  done  in  the  Bernstein  lamp  (see  Fig.  17).  The  carbon 


58      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

filament,  before  inclusion  in  the  bulb,  is  frequently  subjected 
to  a  process  which  is  called  treating.  If  the  carbon  filament 
is  rendered  incandescent  in  an  atmosphere  of  hydro-carbon 
gas — if,  for  instance,  it  is  rendered  incandescent  when  it  is 
immersed  in  coal  gas  or  benzol  vapour — the  heated  carbon 
decomposes  the  hydro-carbon  gas,  and  a  deposit  of  carbon  of 
a  very  dense  form  is  made  upon  the  filament,  the  carbon 


FIG.  17. — Bernstein  Lamp,  with  Straight  Carbon  C,  and  Springs  S  to 
permit  Expansion. 

which  is  so  deposited  being  of  a  graphitic  form,  and  having, 
under  the  proper  conditions  of  deposit,  a  brilliant  steel-like 
lustre.  This  process  is  not,  however,  applied  to  all  carbon 
filaments.  This  deposited  carbon  has  a  lower  electrical  resist- 
ance for  the  same  volume,  viz.,  about  one-sixth  or  seventh, 


ELECTRIC  GLOW  LAMPS.  59 

when  compared  with  the  carbonised  cellulose  of  which  the 
carbonised  parchmentised  thread  is  composed.  In  some  cases 
the  original  carbonisation  of  the  material  is  performed  in 
an  electric  furnace  at  a  very  high  temperature  and  under 
pressure ;  under  these  conditions  the  organic  material  which 
is  carbonised  is  converted  into  an  exceedingly  dense  variety 
of  carbon,  called  adamantine  carbon,  the  surface  of  which  has 
a  smooth  and  polished  appearance  and  a  steel-like  lustre  with- 
out having  been  subjected  to  the  above-described  process  of 
treating.  However  prepared,  the  carbon  filament  has  then  to 
be  attached  to  the  leading-in  wires,  which  are  sealed  through  the 
glass.  These,  as  above  stated,  are  made  of  platinum,  and  no 
really  successful  substitute  has  yet  been  found  for  this.  The 
reason  for  this  selection  is  as  follows  : — In  order  that  the 
wires  by  which  the  current  is  conveyed  to  the  carbon  filament 
may  be  sealed  into  the  glass  airtight,  and  not  shrink  away 
from  the  glass  when  cold,  it  is  necessary  that  the  material  of 
which  the  wires  are  composed  shall  have  practically  the  same 
expansion  as  the  glass,  because  otherwise  the  wires  would 
crack  out  when  the  glass  bulb  was  heated  and  cooled, 
and  the  air  from  outside  would  leak  into  the  vacuum.  It 
happens,  by  a  fortunate  coincidence,  that  the  co-efficient 
of  expansion  of  platinum  is  exactly  the  same  as  that  of  some 
varieties  of  glass,  and  platinum  wires  can  therefore  be  sealed 
through  the  glass  whilst  hot,  and  will  remain  firmly  attached 
to  the  glass  when  cold.  The  carbon  filament  is  fastened  to 
the  ends  of  the  platinum  wires  by  means  of  a  carbon  cement, 
or  a  deposit  of  carbon  made  over  the  junction,  and  the  carbon 
filament  with  its  attached  platinum  wires  is  then  sealed  into 
a  glass  bulb.  We  are  thus  able  to  pass  the  necessary  electric 
current  through  the  filament,  and  yet  preserve  the  carbon  in  a 
highly  perfect  vacuum. 

In  Fig.  18  are  shown  two  lamps  with  carbon  conductors 
of  different  forms.    In  some  cases  the  carbon  filament  has 


60      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

a  simple  horse-shoe  shape  as  originally  adopted  by  Edison ; 
whilst  in  other  cases  it  is  given  a  double  twist,  as  in  the 
Edison -Swan  lamps,  one  or  more  curls  being  used.  For 
some  purposes  it  is  necessary  to  coil  the  filament  into  a  tight 
compact  coil,  in  order  that,  when  the  lamp  is  used  in  the  focus 
of  a  mirror,  the  light  may  be  all  concentrated  in  one  place,  and 


FIG.  18. — Glow  Lamps  with  Zigzag  and  Straight  Carbons 

be  gathered  up  by  the  mirror  or  lens.  Such  lamps  are  called 
focus  lamps,  and  are  employed  in  optical  lanterns,  in  ships' 
side-lights,  and  wherever  it  is  necessary  to  collect  all  the  rays 
from  the  filament  by  optical  means.  These  lamps  are  very 
useful  in  optical  lanterns,  and,  now  that  so  many  houses  are 


ELECTRIC  GLOW  LAMPS. 


61 


supplied  with  electric  current,  afford  an  easy  means  of  show- 
ing lantern  views  in  a  drawing-room.*  In  cases  in  which 
high  candle-power  is  required,  it  is  obtained  by  putting  two  or 
more  filaments  in  the  same  lamp  bulb.  In  Fig.  19  is  shown 
a  representation  of  a  multiple-filament  lamp  of  2,000  candle- 


Fia.  19.— 2,000  candle-power  Glow  Lamp  with  Multiple  Filaments. 

*  Photographic  amateurs  who  practise  the  art  of  making  lantern  slides 
from  negatives,  want  an  easy  and  ready  mode  of  exhibiting  them.  If 
electric  current  is  laid  on  to  the  house,  the  simplest  method  of  doing  this 
is  to  fit  an  ordinary  optical  lantern  with  a  100-volt  focus  lamp  of  50 
candle-power.  This  can  be  fitted  on  the  tray  which  usually  carries  the 
oxyhydrogen  jet.  A  screen  of  drawing  paper  six  feet  square  makes  an 
admirable  surface  for  projection,  and  the  views  can  be  shown  in  a  drawing- 
room  without  danger  or  smell. 


02      .ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

power  as  constructed  by  the  Edison- Swan  Company.  In  these 
lamps  a  number  of  carbon  conductors  of  the  horse-shoe  shape 
are  arranged  between  two  rings,  so  that  the  electric  current 
can  pass  from  one  ring  to  the  other  through  four,  or  five,  or 
six,  or  more  carbon  conductors,  each  of  which  has  one  leg 
attached  to  one  ring  and  the  other  leg  to  the  other.  Lamps 
of  this  high  candle-power  are  made  up  to  3,000  candle- 
power  or  more.  In  such  cases  the  glass  bulbs  become  globes 
of  glass  nearly  a  foot  in  diameter,  and  have  to  be  made  of 
considerable  thickness  to  withstand  the  enormous  pressure  of 
the  air  on  the  outside.  The  large  lamp  now  shown  to  you  in 
action  has  a  globe  having  a  surface  of  280  square  inches,  and 
the  air  pressure  on  the  exterior  amounts  to  one  and  three- 
quarter  tons  !  One  limitation  to  the  safe  size  of  such  lamps 
is  this  great  external  air  pressure. 

The  final  stage  of  manufacture  of  the  lamp  is  the  exhaus- 
tion of  the  lamp  bulb.  It  was  mentioned  above  that  this 
was  rendered  essential,  in  the  first  place,  by  the  fact  that,  if 
heated  in  the  air  to  a  high  temperature,  the  carbon  filament 
would  soon  be  destroyed  by  combustion.  Taking  a  non- 
exhausted  lamp,  I  pass  an  electric  current  through  the 
filament.  You  see  that  the  lamp  burns  for  a  few  seconds 
and  then  goes  out.  The  oxygen  of  the  air  in  the  bulb  has 
combined  with  the  carbon  of  the  filament  in  one  or  more 
places  and  destroyed  it  by  producing  combustion  of  the 
material.  But  if  we  take  a  filament  of  carbon  not  enclosed 
in  a  bulb  and  plunge  it  into  a  glass  vessel  full  of  a  non- 
oxidising  atmosphere,  such  as  carbonic  acid  gas  or  coal  gas, 
and  heat  it  electrically,  you  will  see  that  the  carbon,  though 
rendered  incandescent,  is  not  then  rapidly  destroyed.  The 
carbon  filament  is  enclosed  in  a  vacuum,  not  merely  to  pre- 
vent its  combustion,  but  there  is  another  important  reason 
for  exhausting  the  bulb  of  all  gases  or  air.  Even  if  we  suppose 
the  bulb  filled  with  a  non-oxidising  atmosphere  of  nitrogen  or 


ELECTPJC  GLOW  LAMPS.  63 

hydrogen,  such  a  lamp  would  yet  not  be  so  perfect  as  a  vacuum 
lamp,  and  for  the  following  reason : — Modern  physical 
research  has  provided  arguments  well  nigh  irresistible  to  prove 
that  gases  consist  of  molecules  or  little  particles  which  are 
in  rapid  motion.  This  kinetic  theory  of  the  structure  of  gases, 
as  it  is  termed,  is  supported  by  an  immense  body  of  physical 
evidence,  and  no  less  an  authority  than  Lord  Kelvin  has  called 
it  one  of  the  surest  articles  of  the  scientific  creed.  Certain 
lines  of  investigation  have  shown  how  to  determine  the  average 
velocity  of  these  gas  particles.  It  has  been  shown,  for 
instance,  that  in  hydrogen  gas  at  ordinary  pressure  and  at  the 
temperature  of  melting  ice  the  molecules  are  moving  with  an 
average  velocity  of  about  6,000  feet  per  second,  or  69  miles 
per  minute,  and  in  oxygen  with  an  average  velocity  of  18  miles 
per  minute.  These  gas  molecules  in  their  rapid  flight  collide 
against  one  another  in  moving  to  and  fro,  and  the  average 
space  over  which  the  molecule  flies  before  it  has  a  collision 
against  some  other  molecule  is  called  its  mean  free  path. 

In  one  cubic  foot  of  air  or  oxygen  aft  the  ordinary 
pressure,  and  at  the  temperature  of  melting  ice,  there  are, 
in  all  probability,,  a  number  of  gas  molecules  represented 
numerically  by  a  billion  times  a  billion,  or  by  unity  followed 
by  twenty-four  cyphers  (1024).  The  mean  free  path  of  the 
oxygen  gas  molecule  is  rather  more  than  two-millionths  of  an 
inch,  and  the  average  number  of  collisions  during  the  one 
second  in  which  it  darts  over  a  distance  in  all  of  1,500ft.  is 
7,600  millions.  We  are  here  dealing  with  minute  portions 
of  time  and  space,  in  which  the  unit  is  not  a  second  and 
an  inch,  but  a  millionth  part  of  each  of  these.  The 
millionth  of  an  inch  is  about  the  same  fraction  of  an  inch 
that  one  foot  is  of  200  miles ;  and  the  millionth  part  of  a 
second  is  the  same  fraction  of  a  second  that  one  second  is  of 
about  twelve  days.  All  the  air  we  breathe  and  feel  consists, 
therefore,  of  these  flying  and  colliding  oxygen  and  nitrogen 


04      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

molecules.  Each  particle  or  molecule  is  always  having  its 
direction  changed  by  its  collisions  against  neighbours,  just 
as  a  billiard  ball  has  when  it  "cannons"  against  another. 
It  is  the  constant  bombardment  of  these  molecules  against 
the  walls  of  a  containing  vessel  which  creates  the  elastic 
pressure  of  a  gas,  and  when  any  solid  body  is  surrounded 
by  a  gas  an  enormous  number  of  collisions  take  place 
against  that  surface  in  every  second  of  time. 

If  the  surface  is  heated,  the  molecules  which  come  up 
against  it  take  heat  from  it  in  their  myriads  of  contacts  per 
second,  and  this  removal  of  heat  from  the  body  by  the  gas 
molecules  is  called  convection.  If  the  carbon  filament  is 
enclosed  in  a  glass  bulb  not  exhausted  of  its  air,  a  certain 
amount  of  energy  is  thus  removed  from  the  filament  and 
given  up  to  the  glass,  being  conveyed  by  these  molecular 
vehicles.  The  energy  so  abstracted  from  the  filament  is  not 
in  any  way  conveyed  to  the  eye  as  light.  Hence,  for  all 
illuminating  purposes,  it  is  lost,  and  a  lamp  so  constructed 
with  a  non-vacuous  bulb  cannot  in  some  respects  be  as 
efficient  a  device  as  one  with  a  high  vacuum,  because  of  this 
additional  loss  of  energy  from  the  filament. 

The  process  adopted  for  removing  the  air  from  the  bulb  is 
generally  the  employment  of  some  form  of  mercury  pump  in 
combination  with  a  mechanical  air-pump.  Time  will  not  per- 
mit me  to  describe  the  many  forms  of  this  device  which  have 
been  suggested.  Briefly  speaking,  the  principle  upon  which 
many  of  these  mercury  pumps  act  is  as  follows : — A  vertical 
glass  tube  F  (see  Fig.  20)  has  a  side  tube  S  sealed  into  it,  to 
which  is  attached  the  lamp  to  be  exhausted.  Drops  of  mercury 
are  allowed  to  fall  down  the  vertical  tube,  and  as  they  fall  down 
they  push  the  air  in  the  form  of  bubbles  before  them.  The 
air  is,  therefore,  gradually  drawn  out  from  the  lamp  bulb  L, 
and  the  completion  of  the  process  of  exhaustion  is  recognised 


ELECTEIG  GLOW  LAMPS. 


65 


when  the  globules  of  mercury  fall  down  the  vertical  tube  in 
close  contact  with  one  another  without  carrying  any  air 
between  them.  Under  these  circumstances,  if  the  bottom 
end  of  the  vertical  fall  tube  is  placed  in  a  vessel  of  mercury  V, 
the  mercury  will  gradually  rise  in  the  fall  tube  to  the  height 


Fie.  20. — Diagram  of  arrangements  of  a  Mercury  Pump  for  Exhausting 

Lamps. 

M    Mercury   Reservoir  ;     F,    Fall   Tube  ;     L,  Lamp  being  Exhausted  ; 
S,  Side  Tube  ;  V  Vessel  of  Mercury. 

of  the  mercury  barometer — that  is,  to  about  30in. — and  then 
each  succeeding  drop  of  mercury  which  falls  down  on  to  the 
top  of  the  column,  does  so  with  a  sharp  click,  thus  showing 


66      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

that  the  lamp  bulb  has  been  exhausted  of  its  air  to  a  very 
high  degree.  It  is  not  difficult  to  abstract  from  such  a  glass 
lamp  bulb  all  but  one-millionth  of  the  air  originally  contained 
in  it.  A  highly  exhausted  space  of  this  kind  is  commonly 
called  a  vacuum ;  nevertheless,  when  we  realise  that  every 
cubic  foot  of  air  at  ordinary  pressure  contains  a  billion 
times  a  billion  molecules  of  air,  it  is  easily  seen  that  the 
reduction  of  the  number  of  molecules  which  may  be 
contained  in  a  lamp  bulb  of  about  three  inches  in  diameter 
before  exhaustion,  to  one-millionth  of  their  original  value, 
still  leaves  included  in  the  bulb  many  millions  of  millions 
of  molecules.  One  important  change  that  is,  however, 
made  in  the  physical  state  of  the  gas  when  so  reduced  to 
a  very  low  pressure  is  that  the  mean  free  path  of  the 
molecules  is  enormously  increased.  By  this  reduction  in 
pressure,  the  mean  free  path  of  the  molecules  in  space,  on 
an  average,  traversed  before  collision,  is  increased  just  in 
proportion  as  the  pressure  is  diminished. 

By  the  reduction  of  the  pressure  to  one-millionth  of  its 
normal  value  by  the  abstraction  of  999,999  millionths  of 
the  air,  the  mean  free  path  of  the  air  molecules  is  increased 
a  million- fold — in  other  words,  to  something  not  far  from 
two  inches.  In  the  interior,  therefore,  of  a  large  glow 
lamp,  which  may  have  a  volume,  say,  of  ten  cubic  inches, 
there  is  good  reason  to  believe  that,  when  the  bulb  is 
exhausted  to  the  highest  point  usually  obtainable  in  practice, 
there  still  remain  an  immense  number  of  molecules  of  air, 
but  that  the  space  is,  comparatively  speaking,  so  sparsely 
populated  with  molecules  that  each  molecule  in  flying  to  and 
fro,  may  move,  on  an  average,  over  a  distance  of  an  inch  or  two 
without  collision  against  a  neighbour.  The  rarefaction  of 
the  air  must,  therefore,  decrease  immensely  the  rate  at  which 
energy  is  taken  from  the  filament  by  convection,  because  it 
decreases  the  number  of  molecular  bombardments  against  the. 


ELECTEIC  GLOW  LAMPS.  .  C7 

filament,  and  this  is  one  object  of  producing  the  exhaustion. 
When  the  proper  vacuum  has  been  obtained  in  the  lamp,  the 
glass  inlet  tube  is  melted,  and  the  lamp  is  sealed  off  from  the 
pump.  The  lamp  is  finished  by  adding  to  it  a  brass  collar 
with  two  sole-plates  of  brass,  which  are  fastened  on  to  the 
ends  of  the  platinum  wires  which  protrude  through  the  glass. 
In  this  way  an  electric  current  can  be  sent  through  the 
filament,  and  yet  the  highly  perfect  vacuum  in  the  glass 
bulb  be  indefinitely  preserved. 

Lamps  so  made  may  take  numerous  forms.     In  Fig.  21 
is   shown  a  form  of  lamp  called  a  candle-lamp,  employed 


FlG.  21. — Electric  Candle  Lamp,  of  which  the  Glass  Bulb  is  shaped  like  a 
Candle  Flame. 


for  decorative  and  other  purposes.  Lamps  may  be  made 
so  small  that  they  can  be  employed  for  theatrical,  decorative, 
microscopic,  dental,  and  surgical  purposes,  in  which  case  they 
called  micro-lamps  ;  they  may  be  made  so  large,  and 


are 

have  so  many  carbon  conductors  in  them,  that  they  can  yield 
a  candle-power  of  two  to  three  thousand,  and  the  glass  bulbs 
may,  in  both  cases,  be  given  any  requisite  form  and  colour. 

Having  thus  briefly  described  the  process  of  the  construction 
of  a  glow  lamp,  we  have  now  to  study  a  little  more  closely 

F2 


68      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

the  laws  which  govern  its  operation.  From  what  ha^  been 
already  stated  in  the  First  Lecture,  it  will  be  seen  that,  in 
order  to  possess  a  full  knowledge  of  the  behaviour  of  the 
lamp  as  an  energy-translating  device,  it  is  necessary  to 
know  four  things  about  it.  First,  we  must  know  the 
current,  in  amperes,  which  is  taken  by  the  lamp  when  a 
certain  electric  pressure  is  produced  between  the  ends  of  the 
carbon  filament.  Generally  speaking,  lamps  are  denomi- 
nated by  the  pressure  at  which  they  must  be  worked  to 
give  satisfactory  results  as  to  duration.  This  pressure 
measured  in  volts  is  called  the  marked  volts  or  maker's  volts, 
and  lamps  may  be  distinguished  as  100-volt  lamps,  50-volt 
lamps,  10-volt  lamps,  &c.  The  current  taken  by  a  lamp 
can  be  measured  by  many  kinds  of  instruments,  which  are 
called  ampere-meters  or  ammeters.  The  pressure  between 
the  terminals  of  the  lamp  can  be  measured  by  suit- 
able voltmeters.  Approximately  speaking,  a  lamp  intended 
to  work  at  100  volts  and  to  give  16  candles  light  has  a 
carbon  filament  about  five  inches  long  and  about  one  hundredth 
of  an  inch  in  diameter,  and  when  worked  at  a  pressure  of 
100  volts  it  takes  from  f  to  §  of  an  ampere.  From  explana- 
tions previously  given  it  will  be  remembered  that  if  these 
two  numbers — namely,  the  number  representing  the  volts 
and  the  number  representing  the  amperes — are  multiplied 
together,  we  have  a  number  representing  the  electrical  power, 
estimated  in  watts,  which  is  being  taken  up  in  the  lamp,  and 
such  a  16 -can die-power  100-volt  lamp  generally  takes  from 
50  to  60  watts.  We  have  already  described  the  methods 
by  which  the  candle-power  can  be  measured.  If  we  divide 
the  number  representing  the  electrical  power  in  watts  taken 
by  the  lamp  by  the  number  representing  the  candle-power 
of  the  lamp,  the  quotient  gives  us  the  number  representing 
the  watts  per  candle-power.  In  most  modern  lamps  this  last 
number  varies  from  2J  to  3J,  when  the  lamp  is  being  used 
at  the  marked  volts.  There  are,  therefore,  four  quantities  in 


ELECTRIC  GLOW  LAMPS. 


69 


connection  with  every  lamp  which  it  is  important  to  know. 
First,  the  marked  volts  or  voltage  of  the  lamps;  second, 
the  ampere  current  taken  by  the  lamp  at  the  marked  volts  ; 
third,  the  candle-power  of  the  lamp  ;  and,  fourth,  the  ivatts 
per  candle-power.  Very  briefly  we  may  describe  the  process 
by  which  these  four  quantities  are  determined. 

We  have  already  described  the  instrument  called  an  electro- 
static voltmeter,  which  is  used  for  measuring  electric  pressure. 


FIG.  22.— Simple  Form  of  Ampere-Meter  for  Measuring  Lamp  Currents. 

One  simple  form  of  instrument,  called  an  ammeter,  which 
may  be  used  for  measuring  the  current  in  amperes  taken 
by  a  lamp,  is  made  as  follows  : — A  coil  of  wire  C  (see  Fig.  22) 
is  wound  on  a  bobbin,  and  through  this  coil  of  wire  the 
current  to  the  lamp  is  passed,  This  current  creates  a  mag- 
netic field  round  the  wire,  as  described  in  our  First  Lecture. 
If  a  piece  of  iron  wire  I  is  suspended  on  the  end  of  a 
delicately-balanced  lever,  so  that  the  iron  is  held  freely  just 
at  the  entrance  of  the  bobbin,  the  passage  of  an  electric 
current  through  the  bobbin  will  attract  the  iron  into  the  coil. 


70     ELECTRIC  LAMPS  AND  ELECTRIC 

This  movement  of  the  iron  is  resisted  by  a  weight,  W,  properly 
placed  on  the  balance  arm,  or  by  the  weight  of  an  indicating 
needle.  Such  an  instrument  may  be  graduated  to  show  on 
a  divided  scale  the  strength  of  the  current  in  amperes  which 
is  passing  through  the  coil. 

It  is  important  that,  in  thus  making  a  determination  ol 
the  constants  of  a  lamp — viz.,  the  current  passing  through 
the  lamp,  the  pressure  difference  at  the  terminals  of  the  lamp, 
and  the  candle-power  of  the  lamp  by  comparison  with  a 
suitable  standard  of  light — these  quantities  should  all  be  mea- 
sured at  the  same  instant.  By  varying  the  current  through  the 
lamp,  as  we  can  do  by  using  pressures  of  different  values,  we 
"  can  obtain  a  series  of  observations  in  which  the  candle-power, 
current,  and  volts  on  the  terminals  of  the  lamp  are  all  measured 
simultaneously  for  different  values  of  the  pressure  on  the  lamp 
terminals,  and  these  values  can  be  set  out  in  a  series  of  curves 
which  are  called  the  characteristic  curves  of  the  lamp.  In 
Fig.  23  is  shown  a  curve  illustrating  the  manner  in  which  the 
candle-power  of  a  16-c.p.  lamp  varies  with  the  current.  It 
will  be  seen  that  a  very  small  increase  in  the  current  is 
accompanied  by  a  large  increase  in  the  light  of  the  lamp, 
and  it  can  be  shown  that  the  candle-power  varies  very 
nearly  as  the  sixth  power  of  the  current.  That  is  to  say, 
if  a  lamp  has  an  illuminating  power  equal  to  one  candle 
when  a  current  of  one  ampere  is  passing  through  it,  then 
it  would  give  2x2x2x2x2x2  =  64  candle-power  if  two 
amperes  were  passed  through  it.  No  carbon  filament  lamp 
could  endure  without  destruction  an  increase  in  current  to 
the  extent  of  100  per  cent.,  but  within  narrower  limits 
the  above  law  holds  good,  viz.,  that  the  candle-power  varies 
approximately  as  the  fifth  or  sixth  power  of  the  current. 
In  Fig.  24  is  shown  another  curve,  illustrating  the  manner 
in  which  the  candle-power  varies  with  the  volts,  and  in 
Figs.  25  and  26  curves  showing  the  manner  in  which  the 
candle-power  varies  with  the  watts  and  with  the  watts  per 


ELECTRIC  GLOW  LAMPS. 


71 


20 


t. 


35 


40 


45 


50  55 

Amperesj 


60 


•70 


FIG  23, — Curve  showing  Variation  of  Candle-power  with  Current  in  an 
Electric  Glow  Lamp. 


25 


20 


70 


80 
Volts. 


100 


110 


FIG.  24. — Curve  showing  Variation  of  Candle-power  with  Voltage  in 
an  Electric  Glow  Lamp. 


72      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


candle-power.  From  the  curve  in  Fig.  25  it  will  be  seen  that 
the  candle-power  of  the  lamp  varies  very  nearly  as  the  cube 
of  the  total  power  in  watts  supplied  to  it.  In  other  words 
if  we  supply  the  lamp  with  twice  the  electrical  power,  the 
candle-power  is  increased  eight  times.  If  we  supply  it  with 
three  times  the  electrical  power,  it  is  increased  twenty- seven 
times.  These  curves  will  show,  amongst  other  things,  the 
importance  of  preserving  the  pressure  at  the  terminals  of  a 
carbon  filament  electric  lamp  constant  when  used  as  an 


125 


^ 
O 


C15 

0> 

1 


°£ 


I 


40  50 

Watts. 


60 


70 


80 


FlG.  25. — Curve  showing  Variation  of  Candle-power  with  Wattage 
in  a  Glow  Lamp. 


illuminating  agent.  Any  variation  of  the  pressure,  however 
small,  produces  a  very  serious  variation  in  the  candle-power 
of  the  lamp.  It  is  convenient  to  bear  in  mind  the  following 
figures  for  the  16-c.p.  lamp  as  generally  made.  Such  a  lamp 
takes,  when  worked  at  a  pressure  of  100  volts,  0-6  or  J  of  an 
ampere,  absorbs  60  watts  of  power,  and  therefore  in  16 £  hours 
consumes  an  amount  of  energy  which  is  16|  times  60  =  1,000 


ELECTRIC  GLOW  LAMPS.  73 

watt-hours.  This  amount  of  energy  is  called  one  Board  of 
Trade  Unit,  and  is  generally  sold,  by  the  Electric  Supply 
Companies  in  England  at  prices  varying  from  3d.  to  8d. 

As  soon  as  carbon  filament  electric  lamps  came  into 
general  use  it  was  found  that  lamps  are  like  human  beings 
— they  have  a  certain  life  or  duration,  and  moreover  most 
of  them  undergo  a  process  of  deterioration  in  light-giving 


25 


20 


15 


10 


10  16  20 

Watts  per  Candle 


26 


30 


35 


FIG.  26.— Curve  showing  the  Variation  of  Candle-power  with  Watts  per 
Candle-power  in  an  Electric  Glow  Lamp. 


power.  After  a  certain  time  the  filament  of  the  lamp  is 
destroyed,  and  the  lamp  ceases  to  work.  The  duration  of  any 
lamp  when  worked  under  certain  conditions  as  to  pressure 
and  supply  is  called  its  life,  and  from  a  large  number  of 
observations  of  similarly-constructed  lamps  when  worked  in 
a  particular  manner  we  can  deduce  the  average  li/e  of  a  lamp. 
We  cannot  predict  from  observations  of  one  or  two  lamps 


74      ELECTRIC  LAMPS  AND  ELECTHIC  LIGHTING. 

what  the  average  duration  will  be,  any  more  than  we  can  tell 
from  observations  on  one  or  two  human  lives  what  is  the 
average  duration  of  human  life  in  any  place.  The  life  of 
any  particular  individual  lamp  may  be  only  a  few  hours,  or  it 
may  be  many  thousands  of  hours.  We  shall  presently  point 
out  that  this  average  duration  is  only  one  factor  in  determin- 
ing the  real  value  of  the  lamp  as  an  energy-translating 
device.  Apart  from  accidental  circumstances,  the  factors 
that  determine  the  life  of  the  lamp  are  the  temperature  at 
which  the  carbon  filament  is  kept,  and  the  method  of 
its  manufacture.  Assuming,  however,  that  an  incandescent 
lamp  is  supplied  with  electric  current  at  a  constant  electric 
pressure :  it  is  always  found  that  two  marked  changes  occur 
in  the  lamp  in  the  process  of  time ;  first,  the  lamp  begins 
to  blacken  by  a  deposit  of  carbon  which  is  made  upon  the 
interior  of  the  bulb ;  and,  second,  the  carbon  filament 
undergoes  a  change  by  which  its  electrical  resistance  becomes 
increased  and  its  surface  altered,  so  that,  whatever  its  initial 
condition,  its  surface  finally  assumes  a  much  darker  and  more 
sooty  appearance  than  when  first  manufactured. 

From  these  physical  changes  it  follows  that  the  candle-power 
of  the  lamp  is  diminished,  first  by  an  obstruction  of  light  by 
the  black  deposit  of  carbon  on  the  inner  surface  of  the  glass, 
and,  second,  owing  to  the  increase  in  the  resistance  of  the 
filament,  the  lamp  taking  less  current  and  therefore  furnishing 
less  light.  The  deposit  of  carbon  on  the  interior  of  the  bulb 
is  produced  partly,  or  perhaps  generally,  by  a  process  which 
is  an  ordinary  evaporation  of  the  carbon,  but  it  may  be  pro- 
duced also  by  volatilisation  of  condensed  hydrocarbons  from 
the  interior  of  the  conductor.  In  addition,  however,  to  this, 
there  is  a  process  of  projection  of  carbon  molecules  from  the 
filament  in  straight  lines  which  is  not  of  a  kind  generally 
included  in  the  term  volatilisation.  Many  blackened  carbon 
lamps  show  lines  of  no  deposit  on  the  glass,  which 


ELECTRIC  GLOW  LAMPS. 


75 


have  been  called  molecular  shadows.  It  is  not  unusual  to 
find  lamps  the  interior  surface  of  the  glass  of  which  has 
become  very  black  by  a  deposit  of  carbon.  On  examining  the 
lamp  it  will  be  found  that  on  one  side  of  the  glass,  lying  in 
the  plane  of  the  loop,  there  is  a  line  of  clear  glass  on  which 
no  deposit  has  been  made.  This  is  very  often  shown  well  in 
lamps  in  which  the  filament  has  been  worn  away  at  a  point 
about  halfway  down  one  leg  of  the  loop  (see  Fig.  27) ;  and 
on  examining  such  a  lamp  it  will  be  found  that  there  is  a 
well-marked  line  of  no  deposit  upon  the  glass,  just  in  the 


FIG.  27.— Glow  Lamp  with  the  glass  bulb  blackened  by  deposit  of  carbon, 
molecular  scattering  having  taken  place  from  the  point  marked  a  on  the 
filament,  and  a  shadow  or  line  of  no  deposit  thus  produced  at  6  on  the  glass 


receiver. 


plane  of  the  loop,  and  on  the  side  farthest  removed  from  the 
point  of  rupture.  Blackened  lamps  of  this  kind,  therefore, 
indicate  clearly  that  carbon  molecules  have  been  shot  off 
in  straight  lines  from  the  place  where  the  carbon  has  subse- 
quently been  ruptured.  If  in  a  carbon  filament  there  happens 
to  be  a  weak  spot  or  place  of  high  resistance,  at  that  point 
the  current  will  generate  heat  at  a  greater  rate  than  at  other 


76      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

places,  and  the  temperature  of  that  spot  will  rise  above  the 
temperature  of  the  rest  of  the  filament.  Such  points  of  greater 
temperature  can  often  be  seen  on  carefully  examining  an  old 
or  bad  filament.  From  these  high-temperature  spots,  carbon 
molecules  must  be  projected  off  by  an  action  which  may  be 
partly  electrical,  and  are  deposited  upon  the  glass  on  all  sides. 
But  it  is  clear  that  the  unbroken  leg  would  shield  part  of  the 
glass  from  this  carbon  molecule  bombardment,  and  a  shadow 
of  one-half  of  the  loop  would  therefore  be  formed  upon  the 
glass. 

This  shielding  action  may  be  illustrated  by  a  simple  experi- 
ment.    Here  is  a  f|  -shaped  rod  (Fig.  28) .    This  shall  represent 


FIG.   28.— "Spray  Shadow"  of  a  rod  thrown  on  Cardboard  Screen  to 
illustrate  formation  of  molecular  shadow  in  Glow  Lamps. 

the  carbon  conductor  in  the  lamp,  and  this  sheet  of  cardboard 
placed  behind  it  the  side  of  the  glass  bulb.  I  have  affixed  a 
little  spray-producer  to  one  side  of  the  loop,  and  from  that 
point  blow  out  a  spray  of  inky  water.  Consider  the  ink  spray 
to  represent  the  carbon  atoms  shot  off  from  the  overheated 
spot.  We  see  that  the  cardboard  is  bespattered  on  all  points 
except  along  one  line  where  it  is  sheltered  by  the  opposite 
side  of  the  loop.  We  have  thus  produced  a  "spray-shadow  " 
on  the  board.  The  existence  of  these  molecular  shadows 


TJNH 


ELECTRIC  GLOW  LAMPS.  77 

in  incandescent  lamps  leads  us,  therefore,  to  recognise 
that  the  carbon  atoms  must  be  shot  off  in  straight  lines,  or 
else,  obviously,  no  such  sharp  shadow  could  thus  be  formed. 
It  has  been  asserted  by  some  writers  that  the  whole  of  the 
black  coating  on  the  interior  of  the  carbon  glow  lamps  is 
produced  solely  by  a  process  of  evaporation  of  carbon  ;  but 
the  frequent  existence  of  these  shadows  on  blackened  lamps 
show  that  under  some  conditions  the  projection  of  carbon 
from  the  filament  is  not  merely  a  general  and  irregular 
volatilisation,  but  a  copious  projection  of  carbon  molecules, 
which  move  out  in  perfectly  straight  lines  from  one  part  of 
the  filament.* 

It  is  clear,  therefore,  from  all  the  foregoing  considerations 
connected  with  the  processes  which  are  going  on  in  the 
interior  of  the  lamp  and  the  resulting  ageing  of  the  filament, 
and  from  the  changes  that  take  place  in  the  candle-power  with 
change  of  current,  that  the  following  conditions  must  be 
observed  in  order  to  secure  the  best  results  in  the  employment 
of  incandescent  lamps  for  illuminating  purposes.  In  the 
first  place,  the  electric  pressure  which  is  supplied  to  the 
consumer  must  be  exceedingly  constant.  Where  a  supply 
is  drawn  from  the  mains  of  a  public  electric  lighting  company 
the  consumer  will,  usually,  have  no  control  over  the  pressure, 
at  any  rate  in  the  case  of  continuous  currents.  The  public  sup- 
ply companies  of  Great  Britain  are  bound  by  their  Electric 

*  The  author  possesses  a  large  collection  of  lamps  showing  these 
molecular  shadows.  In  the  case  of  the  old  Edison  lamp,  the  bamboo 
filament  was  attached  to  the  platinum  leading-in  wires  by  a  deposit  of 
copper  made  over  the  clamp.  Under  these  conditions,  if  one  clamp 
became  loose,  or  made  a  bad  contact,  a  scattering  of  copper  molecules  took 
place  from  that  clamp,  producing  a  green  transparent  deposit  of  copper 
over  the  interior  of  the  bulb,  which  deposit  generally  showed  a  well- 
marked  shadow  of  one-half  of  the  carbon  loop  upon  the  glass.  Shadows 
have  also  been  produced  by  the  author  by  the  volatilisation  of  aluminium 
plates  in  lamps,  the  volatilisation  having  been  created  by  passing  a  sudden 
strong  current  through  an  aluminium  wire  or  plate  included  in  the  lamp 
bulb.  This  aluminium  deposit  is  of  a  fine  blue  colour. 


78      ELECTEIC  LAMPS  AND  ELECTRIC  LIGHTING. 

Lighting  Orders  to  furnish  current  at  a  certain  constant  pres- 
sure, which  is  generally  100  or  110  volts,  and  conditions  are 
laid  down  by  the  British  Board  of  Trade  that  the  pressure 
shall  not  vary  more  than  four  volts  above  or  four  volts  below 
this  specified  pressure.  These  are  very  wide  limits  of  varia- 
tion, and  very  few  of  the  supply  companies  avail  themselves 
of  them.  The  consumer  who  takes  current  from  a  public 


FIG.  29.— Holden's  Self-recording  Hot- Wire  Voltmeter. 


supply  company  should  always  take  pains  to  ascertain  for  him- 
self whether  the  pressure  is  constant.  He  can  do  this  by 
the  employment  of  a  voltmeter,  such  as  Lord  Kelvin's  electro- 
static voltmeter,  described  in  the  previous  Lecture,  Fig.  8,  or 
he  can  employ  a  self-registering  voltmeter,  of  which  there  are 
many  in  use,  which  will  record  upon  a  drum  covered  with 


ELECTRIC  GLOW  LAMPS.  79 

paper  a  curve  indicating  the  variation  of  pressure  during  the 
day  and  night.     Two  of  these  recording  voltmeters   which 
are  now  much  in  use  are  those  devised  by  Prof.  G.  Mengarini 
and  Captain  Holden.     In  the  latter  instrument  (see  Fig.  29), 
a  very  fine  wire  is  connected  across  between  the  two  points 
between  which  pressure  has  to  be  determined.     A  small  cur- 
rent flows  through  this  wire  and  heats  it,  causing  it  to  expand. 
The  expansion  of  this  wire  is  then  made  to  move  a  lever, 
which,  by  means  of  a  writing  pen,  records  upon  a  drum  driven 
by  clockwork  a  curve,  and  any  variation  in  the  pressure  or 
voltage  is  shown  by  the  form  of  the  line   so   drawn.     The 
other  instrument,  due  to  Prof.  Mengarini,  is  of  a   different 
type.     In  this  apparatus  a  current  taken  from  the  two  points 
between  which  the  pressure  has   to  be  measured  is  passed 
through  two  coils  of  wire — one  a  fixed  coil,  and  the  other  a 
coil  suspended  by  two  steel  wires.     The  fixed  and  movable 
coils  are  so  arranged  (see  Fig.  30)  that  when  a  current  passes 
through  the  two  it  causes  the  movable  coil  to  twist  round, 
or  tend  to  twist  round,  into  a  new   position.     This   action 
of  the  current  is  resisted  by  the  suspended  wires.     A  writing 
pen  is  attached  to  the  movable  coil  and  draws  a  curve  upon 
a  revolving  drum,  as  above  described,  and  this  curve  records 
any  variation  in  the  voltage.     The  chart  so  drawn  can  then  be 
examined  at  the  end  of  every  day,  and  a  record  is  thus  kept  of 
the  variation  of  pressure  during  the  twenty-four  hours.    If  the 
consumer  finds  that  the  light  of  his  incandescent  lamps  varies 
a  great  deal,  or  if  the  life  of  the  lamps  is  very  irregular,  it  is 
well  to  have  such  a  curve  of  pressure  automatically  drawn 
by  a  self-recording  voltmeter  for  three  or  four  days,  and  in 
this  way  to  detect  the  limits  within  which  the  electric  pres- 
sure varies.     Consumers  are  very  apt  to  lay  the  blame  imme- 
diately upon  the  lamps  for  any  variation  in  light  or  brevity 
in  life,  but  as  the  electric  pressure  is  partner  with  the  lamp 
in  producing  the  result,  it  is  evident  that  the   sins  of  the 
supply  companies  in  giving  a  variable  pressure  ought  not  to 


80      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

IOQ  laid  upon  the  lamps.  On  the  other  hand,  if  the  recording 
voltmeter  shows  a  great  uniformity  in  electric  pressure,  and 
if  still  the  life  of  the  lamps  is  not  satisfactory,  then  either  the 
lamps  are  in  fault,  or  else  the  consumer  is  not  using  lamps  of 
the  proper  voltage  for  his  circuits.  Before  ordering  lamps,  it 
is  well  to  ascertain  also  in  this  way  what  is  the  actual  pressure 


FIG.  30. — Mengarini's  Self-recording  Voltmeter.* 

on  the  circuits,  and  then  to  order  lamps  of  the  proper  marked 
volts  for  the  particular  pressure.  Incandescent  lamps 
which  have  been  made  and  marked  by  the  maker  to  work  at 
100  volts  are  not  intended  to  be  worked  at  102  or  104,  and,  if 
they  are,  an  abbreviation  in  the  average  life  must  certainly 

*  Reprinted  by  permission  from  the  Society  of  Arts  Journal. 


ELECTRIC  GLOW  LAMPS. 


81 


occur,  or  at  any  rate  a  more  rapid  blackening  of  the  globe  and 
diminution  of  the  candle-power  than  would  be  the  case  if  the 
consumer  employed  lamps  of  the  proper  voltage.  If  the  supply 
is  very  irregular  in  pressure,  it  is  never  possible  to  obtain 
the  same  good  results  in  the  duration  of  the  lamp  and 
uniformity  in  brilliancy  which  can  be  obtained  if  the  supply 
is  maintained  constant  in  pressure,  at  least  within  the  limits 
of  one  or  two  volts. 

Let  us  at  this  point  note  that,  apart  from  the  question  of 
duration   or    cost,   the   physical   value   of  a   glow   lamp   is 


FIG.  31. — Glow  Lamp  Burning  under  Water. 

measured  by  its  utility  as  an  energy-transforming  device. 
An  electric  glow  lamp  is  a  machine  for  transforming  energy 
from  one  form,  viz.,  the  energy  of  an  electric  current,  into 
another  form,  viz.,  eye-affecting  radiation.  As  explained  in 
Lecture  L,  it  transforms  the  electric  energy  into  radiant  energy, 
or  energy  of  ether  waves,  only  a  fraction  of  which  can  impress 
the  eye  as  light.  This  fraction  varies  from  3  to  7  per  cent. 
This  numerical  expression  for  the  "luminous  efficiency  "  can 
be  determined  by  suitable  experiments  in  which  the  relative 


82       ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

amounts  of  luminous  and  dark  heat  sent  out  by  a  glow  lamp 
are  separately  measured.  If  a  glow  lamp  L  (see  Fig.  31)  is 
immersed  in  water  in  a  transparent  vessel  V  the  water  absorbs 
or  stops  nearly  all  the  dark  or  non-luminous  radiation,  but 
permits  a  large  fraction  of  the  luminous  radiation  or  light  to 
pass  out.  By  photometering  the  lamp  both  when  in  and 
when  out  of  the  water  the  fraction  of  the  luminous  radiation 
which  is  absorbed  can  be  determined.  By  measuring  the  rise 
of  temperature  of  the  water  which  is  created  by  the  absorption 
of  the  radiation,  both  when  the  luminous  radiation  is  allowed 
to  pass  out  and  when  it  is  wholly  stopped  by  blackening  the 
lamp,  we  can  determine  the  ratio  between  the  amount  of  the 
radiation  which  affects  the  eye  as  light  and  that  which  cannot 
affect  it  at  all.  A  well-devised  series  of  experiments  of  this 
kind  have  been  carried  out  by  Mr.  E.  Merritt,  at  Cornell 
University,  U.S.A.  The  result  has  been  to  show  that,  on 
an  average,  about  5  per  cent,  of  the  energy  supplied  to  the 
glow  lamp  as  ordinarily  used  is  utilized  to  make  light,  and 
the  other  95  per  cent,  is  non-effective  as  far  as  the  eye  is 
concerned.  Hence,  if  a  glow  lamp  is  supplied  with  60  watts 
and  gives  16  candles  light,  only  3  watts  are  really  transformed 
into  light,  and  the  mechanical  value  of  the  light  so  produced 
is  rather  less  than  a  quarter  of  a  watt  per  candle. 

This  luminous  efficiency  varies  with  the  temperature  of  the 
filament.  At  a  low  red  heat  it  is  not  more  than  1  per  cent., 
but  increases  to  about  5,  6,  or  7  per  cent,  at  the  normal 
working  temperature.  Thus,  for  an  Edison  16-c.p.  lamp 
Mr.  Merritt  found  the  following  figures  when  the  lamp  was 
worked  at  various  pressures  so  as  to  give  different  candle- 
power. 

It  will  be  seen  that  the  luminous  efficiency  increases  as 
the  "watts  per  candle-power"  decreases — that  is,  as  the 
temperature  of  the  filament  increases. 


ELECTRIC  GLOW  LAMPS. 


83 


The  "  luminous  efficiency  "  must  be  carefully  distinguished 
from  that  which  is  sometimes  called  "the  efficiency"  of  the 
lamp,  and  which  is  the  reciprocal  of  the  watts  per  candle- 
power  or  the  candle-power  yielded  per  horse-power  or  per 
watt  expended  in  the  filament. 

Observations' on  the  Luminous  Efficiency  of  a  16-c.p.  Edison  Glow  Lamp. 


Working 
volts. 

Candle- 
power. 

Power 
absorbed 
in  watts. 

Watts  per 
candle- 
power. 

Luminous 
radiation 
in  watts. 

Luminous 
efficiency  as  a 
percentage. 

74-2 

0-9 

34-6 

38'0 

0-18 

0'5  per  cent. 

91-6 

48 

56-2 

12-0 

0-68 

1-2 

97-3 

7-3 

64-6 

9-0 

1-13 

1-7 

100-3 

8-9 

69-3 

7-8 

162 

23 

107-6 

14-6 

81-6 

5-6 

2-97 

3-6 

109-3 

16-3 

84-4 

5-2 

457 

5-4 

124-1 

38-2 

115'4 

3-0 

7-46 

6-5      „ 

It  will  be  convenient  at  this  stage  of  our  discussion  to  enter  a 
little  more  fully  into  the  changes  that  go  on  in  an  incandes- 
cent electric  lamp  during  its  life.  We  have  pointed  out 
above  the  causes  which  are  at  work  to  change  the  surface  and 
ultimately  to  destroy  the  carbon  filament.  These  causes  com- 
mence to  act  from  the  very  moment  when  the  lamp  begins  to 
be  used.  If  a  carbon  filament  electric  glow  lamp  is  placed 
upon  a  circuit  of  perfectly  constant  pressure,  and  if  the  con- 
stants of  the  lamp — namely,  the  candle-power,  the  current,  the 
resistance  of  the  filament,  the  power  taken  up  in  watts,  and 
the  watts  per  candle-power — are  measured  at  intervals,  say  of 
every  100  hours,  it  will  be  found  that  the  changes  which  go 
on  in  the  lamp  are  somewhat  as  follows  : — During  the  life  of 
the  lamp  there  is  a  progressive  rise  in  resistance,  a  progres- 
sive diminution  in  candle-power,  and  a  progressive  increase 
in  watts  per  candle-power.  An  illustration  of  these  changes 
is  diagrammatically  shown  for  certain  forms  of  ordinary 
vacuum  lamps  by  the  curves  given  in  Fig.  32.  These  curves 

G  2 


84      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

are  deduced/  not  from  observations  on  one  lamp  alone,  but 
from  the  average  of  a  large  number.  If,  instead  of  working 
the  lamp  at  its  marked  volts  or  normal  pressure,  we  raise  the 
voltage,  say,  10  per  cent,  above  the  normal,  changes  of  the 
same  character  take  place,  only  they  are  very  much  ac- 
celerated, and  the  result  for  a  collection  of  lamps  worked 
at  10  per  cent,  above  the  normal  pressure  is  given  in  Fig.  33. 


10 


14 


£19 


=5  4 

c 


\ 


\ 


7. 


280 


270 


260 


250  6 

.c 
O 

240  - 

0) 

u 

c 

cB 

230« 
to 


220 


210 


100 


200 


300       400 ,      6OO 
Life    m   Hours 


GOO        700        800 


FIG.  32. — Curves    showing    the    increasing  resistance    and    diminishing 
candle-power  of  a  Glow  Lamp  during  its  life. 


On  the  other  hand,  if  we  desire  to  keep  the  candle-power 
always  constant,  we  have  to  continually  raise  the  pressure, 
and  at  the  same  time  we  are  increasing  the  watts  per  candle 
and  the  resistance  of  the  lamp.  These  progressive  changes 
are  illustrated  in  the  two  curves  in  Figs.  34  and  35,  in  the 

*  These  curves  are  borrowed  from  a  very  instructive  Paper  by  Prof. 
Nichols,  of  Cornell  University.  U.S.A.,  on  "The  Artificial  Light  of  the 
Future." 


ELECTRIC  GLOW  LAMPS. 


85 


first  of  which  a  16-c.p.  lamp  was  worked  at  continually- 
increasing  pressures  sufficient  to  keep  the  candle-power 
always  constant,  and  in  the  second  of  which  the  candle-power 
was  raised  to  four  times  its  normal  value  and  kept  constant. 

The  value  of  a  lamp  to  a  user  is  not  to  be  measured  merely 
by  the  watts  per  candle-power  it  takes  at  the  commencement 


3-00 


=5  2-75 


225 


i-  2  00 

<U 


•o 
1 
9  150 


V25     20 


180 


'75 


170 


165  E 

jc 
O 


55 


150 


145 


6 

Life 


8  10 

in   Hours. 


12 


16 


140 


FIG.  33.-  Curves  showing  the  rapid  decrease  in  candle-power  and  in- 
crease in  resistance  in  a  Glow  .Lamp  worked  at  10  per  cent,  above  its 
normal  pressure. 


of  its  life,  but  also,  to  a  .arge  degree,  by  the  extent  to  which 
this  consumption  of  energy  and  production  of  light  remains 
constant  during  the  life  of  the  lamp.  This  is  best  expressed 
by  the  percentage  of  the  variation  of  the  candle-power  during 
the  life  of  the  lamp  when  taken  at  equal  intervals  of  time. 
A  16-c.p.  glow  lamp,  taking  say  60  watts  at  the  commence- 
ment of  its  life,  but  which  after  two  or  three  hundred  hours 


86       ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


in  use  is  giving  no  more  than  8  candles  light,  although  taking 
the  same  power,  is  costing  the  consumer  twice  as  much  per 
candle  at  the  end  of  this  time  as  at  the  beginning.  Some 
writers  have,  therefore,  suggested  that  at  the  end  of  a  certain 
period,  when  the  candle-power  has  suffered  a  definite 
diminution  in  value,  say,  of  more  than  30  per  cent.,  the 
lamp  should  be  ''smashed,"  and  a  new  lamp  put  in  its 
place.  This,  however,  is  not  always  advisable  or  necessary. 


35     120 


34 


22  33 

I 


032 


240 


235 


230 


105 


100,. 


225 


25 


75 


100 


220 


Life    in    Hours. 


FIG.  34. — Curves  showing  the  increase  in  Voltage  required  to  keep  the 
candle-power  of  a  Glow  Lamp  constant. 

If  the  lamp  in  the  position  in  which  it  is  being  used  is  giving 
light  enough  for  the  purpose  for  which  it  is  required,  then, 
although  its  candle-power  may  be  diminished,  if  the  total 
power  taken  by  the  lamp  is  diminished  also,  the  total  sum 
which  is  being  paid  by  the  consumer  for  the  energy  supplied 
to  that  lamp  is  on  the  whole  diminished,  and  therefore  the 
lamp  is  costing  the  consumer  less  on  the  whole  to  maintain 


ELECTRIC  GLOW  LAMPS. 


87 


in  action  than  a  new  lamp  would  do  if  it  replaced  the  old  one, 
although  at  the  same  time  the  cost  of  the  lamp  per  candle- 
light may  be  considerably  increased.  Generally  speaking, 
users  of  electric  glow  lamps  have  an  idea  that  the  lamp  which 
lasts  the  longest  is  necessarily  the  best.  From  what  has 
been  stated  above,  it  will  be  seen  that  it  is  not  merely  long  life, 
but  constancy  of  candle-power,  combined  with  high  luminous 
efficiency  and  low  cost,  which  are  really  the  factors  deter- 


V95 


V90 


£185 


1  80  122 


•o  1  76 


70 


1  66 


160 


128 


126 


124 


20 


118 


116 


114 


20 


40 


60  80          100 

Life   in    Minutes. 


120 


140         160 


136 
134 

132 
in 

130  E 

O 

128B 

I 

o>' 

r 
124. 

122 

120 


FIG.  35.—  Curves  showing  the  increasing  Voltage  necessary  to  keep  the 
candle-power  of  a  Glow  Lamp  constant  and  equal  to  four  times  its  normal 
amount.  '  • 


mining  the  value  of  the  lamp  to  the  consumer.  The  duration 
of  the  lamp  is  intimately  connected,  apart  from  accidents, 
with  the  temperature  at  which  the  carbon  filament  is  main- 
tained, and  this  is  determined  by  the  watts  per  candle-power 
being  expended  in  the  lamp.  Makers  of  glow  lamps  some- 
times make  statements  which  lead  the  uninitiated  to  believe 


88      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

that  there  is  a  certain  "efficiency"  or  peculiarly  low  value 
of  the  watts  per  candle-power  belonging  especially  to  a 
certain  make  of  lamp.  It  should  be,  however,  clearly  under- 
stood that  any  glow  lamp  may  be  worked  at  values  of  the 
watts  per  candle-power  varying  over  very  wide  limits,  but 
experience  has  shown  that  carbon  filaments,  as  at  present 
made,  cannot  be  worked  at  temperatures  higher  than 
those  corresponding  to  about  2  to  2|  watts  per  candle- 
power  without  very  rapidly  being  destroyed. 

A  number  of  tests  have  been  made  by  Messrs.  Siemens  and 
Halske  on  lamps  of  various  types,  and  in  which  groups  of 
these  lamps  were  worked  at  different  watts  per  candle-power 
from  2  to  4  and  upwards.  The  lamps  were  measured  at 
intervals  of  100  hours,  and  the  diminution  in  the  observed 
candle-power  expressed  as  a  percentage  of  the  original  candle- 
power  was  laid  down  in  the  form  of  curves.  The  results 
of  these  tests  seemed  to  show  that,  on  the  whole,  lamps 
worked  at  from  3  to  3  J  watts  per  candle-power  had  a  greater 
average  constancy  in  candle-power  than  those  worked  at  a 
lower  number  of  watts  per  candle-power.  That  is  to  say,  if  we 
take  a  16-c.p.  glow  lamp  and  work  its  filament  at  such  a 
temperature  that  it  is  taking  on  the  whole  32  watts,  or 
2  watts  per  candle-power,  it  may  take  less  power  to  begin 
with,  but  it  will  not  preserve  nearly  the  same  constancy  of 
candle-power  as  if  it  had  been  worked  at  such  a  tempera- 
ture that  it  absorbed  from  48  to  56  watts  at  the  com- 
mencement of  its  career ;  in  other  words,  absorbed  3  to  3^ 
watts  per  candle-power.  No  hard  and  fast  rule,  however, 
can  be  laid  down  about  these  matters.  No  deductions  can 
be  made  which  have  any  great  value  from  tests  made 
upon  a  small  number  of  lamp*.  It  has  already  been  stated 
that  the  lives  of  electric  lamps  are  in  this  respect  like 
human  lives,  and  just  as  the  insurance  companies  have  to 
draw  their  deductions  and  to  make  their  calculations  for 


ELECTRIC  GLOW  LAMPS.  89 

life  insurance  on  the  results  of  figures  obtained  from  a 
very  large  number  of  human  lives,  so  inferences  made 
from  the  results  of  statistical  tests  carried  out  on  incan- 
descent lamps  are  only  of  use  if  they  are  made  on  a  very  large 
number  of  lamps.  Broadly  speaking,  however,  the  duration 
of  an  incandescent  lamp  is  dependent  upon  the  electric 
pressure  at  which  the  lamp  is  worked,  and  if  a  lamp  is  marked 
to  be  used  on  a  100-volt  circuit,  then  if  it  is  used  on  a  circuit 
at  a  pressure  of  105  volts,  its  life  would  be  greatly  abbreviated, 
probably  to  one  quarter  or  one  fifth  of  the  time  which  it  would 
last  if  worked  at  the  normal  pressure.  On  the  other  hand, 
if  it  is  worked  at  a  pressure  of  95  volts,  its  duration,  accidents 
apart,  will  be  very  greatly  increased.  It  is  useful,  therefore, 
to  have,  under  these  circumstances,  a  general  knowledge  of 
what  would  be  the  variation  in  candle-power  if  the  same  lamp 
is  employed  at  different  voltages.  The  tables  on  the  next  page 
give  the  candle-power  yielded  by  two  particular  electric  glow 
lamps  when  worked  at  varying  pressures,  the  first  table  being 
for  a  100-volt  16-c.p.  lamp,  and  the  second  table  being  for  a 
100-volt  8-c.p.  lamp.  A  very  cursory  examination  of  these  tables 
will  show  that  the  light  given  by  the  lamp  varies  enormously 
with  the  pressure,  and  that  a  very  small  diminution  of  pressure 
suffices  to  deprive  the  user  of  a  very  large  fraction  of  the 
candle-power  for  which  the  lamp  is  constructed. 

It  may  not  be  inappropriate  at  this  point  to  call  the  atten- 
tion of  users  of  incandescent  lamps  to  the  fact  that  price  is 
not  the  only  factor  to  be  considered  in  connection  with  the 
choice  of  a  glow  lamp.  Consideration  should  also  be  given 
to  the  rate  at  which  the  candle-power  decays  when  constant 
pressure  is  kept  on  the  terminals  of  the  lamp,  as  well  as  to 
the  initial  efficiency,  and  the  useful  life  of  the  lamp.  The 
useful  life  of  the  lamp,  so  far  as  most  consumers  are  con- 
cerned, must  be  considered  to  be  limited  to  that  period  of  the 
total  lamp  life  in  which  its  candle-power  is  not  diminished  by 


90      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


Tables  showing  'Hie  Candle-power  of  certain  nominal  8  and  16 
Candle-power  Incandescent  Lamps  when  worked  at  Various 
pressures  in  Volts. 


A  16  Candle-power  Lamp. 

An  8  Candle-power  Lamp. 

Worked  at 

Yielded  a  candle- 
power  of 

Worked  at 

Yielded  a  candle- 
power  of 

105  volts. 

22'8  c.p. 

102  volts. 

79  c.p. 

100 

167 

100 

69    „ 

95 

122 

98 

61     „ 

90 

8-7 

96 

535  „ 

85 

5-9 

94 

47     „ 

80 

4-0 

92 

4-2    „ 

75 

25 

90 

3-65  „ 

70 

1-5 

88 

3-2    „ 

65      „ 

0-8     „ 

86 

2-8     „ 

60     „ 

0-6    „ 

84 

2-35  „ 

82 

2-2    „ 

80 

1-7    „ 

78 

1-4    „ 

76 

1-15  „ 

74 

0-9     „ 

more  than  30  to  40  per  cent,  of  its  original  candle-power, 
although  this  is  not  by  any  means  a  definite  limit  to  the 
utility  of  the  lamp.  The  user  has  to  determine  what  is  the 
total  cost  to  him,  during  that  useful  life,  of  the  energy 
supplied  to  the  lamp,  and  what  is  the  total  candle-power 
hours  given  by  the  lamp.  Suppose,  for  example,  that  the 
lamp  is  a  100-volt  16-c.p.  lamp,  and  that  it  takes  originally 
50  watts  to  bring  it  to  this  candle-power,  when  oil  a 
100-volt  circuit.  Then  let  us  suppose,  in  the  course  of 
600  hours'  burning,  that  it  is  reduced  to  8  c.p.,  and  that 
it  then  takes  40  watts  at  the  same  pressure.  If  the  rate 
of  decay  is  tolerably  uniform,  the  total  energy  taken  by 
the  lamp  in  watt-hours  is  the  product  of  45  =  J  (50  +  40), 
and  600,  or  27,000  watt-hours  ;  which  is  27  British  Board  of 
Trade  units  (frequently  written  B.T.U.).  The  average 
candle-power  hours  given  by  the  lamp  is  J  (16 +  8)  =  12, 


ELECTEIC  GLOW  LAMPS.  91 

which  multiplied  by  600  gives  7,200  candle-power  hours. 
If  the  Board  of  Trade  unit  of  electric  energy  costs  6d., 
then  the  total  cost  of  the  energy  taken  up  is  13s.  6d., 
for  which  the  consumer  gets  7,200  candle-hours  of  light  at  a 
mean  candle-power  of  12.  If  the  lamp  originally  cost  Is.  6d., 
the  total  cost  of  the  7,200  candle-hours,  including  lamp,  is 
15s.,  or  the  consumer  gets  40  candle-hours  for  a  penny,  given  at 
a  mean  candle-power  of  12.  In  other  words,  he  is  paying 
O3  of  a  penny  per  hour  for  his  lamp  and  light.  A  penny 
includes,  therefore,  the  cost  of  lamp  and  power  for  about  3J 
hours.  Suppose,  however,  the  user  is  offered  a  lamp  at  the 
price  of  Is.,  but  that  extended  experience  shows  that  a 
16-c.p.  lamp  of  this  kind  falls  to  8  candle-power  in  300  hours' 
use.  It  is  clear  that  this  last  lamp,  if  it  takes  the  same  power 
tit  each  candle-power  as  the  other,  will  transform  in  this  time 
45x300  =  13,500  watt-hours,  or  13  J  Board  of  Trade  units, 
and  the  total  cost  of  lamp  at  Is.  and  power  at  6d.  per 
unit  to  the  user  will  be  7s.  9d.  For  this  he  gets  12  x  300 
=  3,600  candle-hours,  or  39  candle-hours  for  a  penny. 
Hence,  the  cheaper  lamp  has  not  really  much  benefited  him. 

Therefore,  the  question  which  is  so  commonly  asked,  viz., 
Is  electric  light  cheaper  or  dearer  than  gas  ?  cannot  be  an- 
swered by  a  simple  "  Yes  "  or  "  No."  The  factors  which  decide 
the  relative  cost,  so  far  as  the  electric  glow  lamp  is  concerned, 
are  as  follows  : — 1st.  The  price  paid  per  Board  of  Trade  unit 
of  electric  energy.  2nd.  The  price  of  the  lamp.  3rd.  The 
average  useful  life  of  the  lamp.  This  last  will  depend  not 
only  upon  the  lamp  itself,  but  also  upon  the  constancy  of  the 
electric  pressure  of  the  supply.  4th.  The  manner  in  which 
the  "  wiring"  of  the  house  or  building  has  been  done ;  a  factor 
which  greatly  determines  the  degree  to  which  the  consumer 
can  effect  economy  by  not  using  more  lamps  than  is  actually 
necessary.  5th.  The  intelligence  shown  in  placing  the  lamps 
so  as  to  make  each  do  the  utmost  illuminating  duty,  and  the 


92       ELECTUIC  LAMPS  AND  ELECTRIC  LIGHTING. 


care  with  which  the  control  is  effected  by  switching  off 
lamps  not  actually  needed.  We  may  add  also  a  6th.  The 
employment  of  a  by-no-means  negligible  precaution  in 
securing  the  best  results — the  keeping  of  the  outside  of 
the  lamps  clean  and  free  from  dust  and  dirt  by  having  them 
well  washed  at  intervals.  A  dusty  lamp  gives  much  less 
light  than  a  clean  one. 

To  assist  the  reader  in  making  certain  essential  com- 
parisons, a  table  is  given  below  which  shows  the  total 
cost  in  power  and  lamp  renewals  for  working  for  200 
hours  a  16-c.p.  100- volt  glow  lamp  taking  on  an  average 
50  watts.  The  costs  are  given  for  various  prices  of  the 
Board  of  Trade  unit  from  3d.  to  8d.,  and  for  lamps  costing 
Is.  and  Is.  6d.,  and  for  useful  lives  of  from  200  to  1,000  hours. 
Let  it  be  borne  in  mind  that  the  "  useful  life"  of  a  lamp  is 
not  a  sharply  marked  period,  but  is  the,  approximate  average 
time  during  which  its  candle-power  has  not  deteriorated  by 
more  than  say  40  per  cent,  of  its  original  value. 

Table  showing  the  cost  of  200  hours'  use  of  a  100-volt  16-c.p.  Glow 
Lamp,  including  the  proportionate  cost  of  Lamp  lienewals  and 
cost  of  Power.  The  Lamp  is  assumed  to  take  on  an  average 
50  watts,  and  to  cost  either  Is.  or  Is.  6d. 


Cost  of  Energy. 
Board  of  Trade  Unit. 

3d. 

4d. 

5d. 

6d. 

7d. 

8d. 

Price  of  Lamp. 

I/-     1/6 

I/-    1/6 

I/-    1/6 

I/-    1/6 

I/-    1/6 

I/-      1/6 

Useful  Life  of  Lamp 
in  hours. 

Cost  of  200  hours'  use  of  Lamp. 

200  hours. 

3/6    4/- 

4/4  4/10 

5/2    5/8J6/-     6/6 

6/10  7/4 

7/8      8/2 

400       „ 

3/-    3/3 

3/10  4/1 

4/8  4/11 

5/6    5/9 

6/4   6/7 

7/2      7/5 

600       „ 

2/10  3/- 

3/8  3/10 

4/6    4/8 

5/4    5/6 

6/2    6/4 

7/-      7/4 

800       „ 

2/9  2/10 

3/7  3/8 

4/5    4/6 

5/3    5/4 

6/1    6/2 

6/11    7/- 

1,000       „ 

2/8    2/9 

3/6  3/7 

4/4    4/5 

5/2    5/3 

e/.   6/1 

6/10  6/11 

ELECTRIC  GLOW  LAMPS.  93 

The  foregoing  table  must  be  read  as  follows : — Suppose 
the  user  gets  current  at  6d.  per  unit,  and  buys  lamps  at  Is.  6d., 
and  let  the  lamp  he  buys  be  assumed  to  have  an  average  useful 
life  of  800  hours.  On  looking  along  the  column  in  line  with 
800  and  under  the  heading  6d.  and  Is.  6d.,  will  be  seen  5s.  4d. 
This  is  the  cost  of  the  16-c.p.  lamp  for  200  hours  of  burning, 
including  proportion  of  lamp  cost  and  power.  Hence,  in  this 
case  the  800  hours'  burning  cost  21s.  4d.  on  the  whole,  and 
for  this  the  user  gets  an  average  candle-power  of  about  13 
candles  for  800  hours,  or  13  x  800  =  10,400  candle-hours.  It 
is  obvious  that  no  hard  and  fast  comparison  with  the  cost  of 
equivalent  candle-hours  given  by  gas  light  is  possible,  unless 
a  careful  statement  is  made  of  the  manner  in  which  the  two 
lights  are  used.  One  great  advantage  which  electric  illumina- 
tion possesses,  from  a  domestic  and  economical  point  of  view, 
is  the  ease  with  which  it  can  be  turned  off  and  on.  A  switch 
placed  near  the  door  enables  a  person  leaving  a  room  to 
turn  out  all  the  electric  lights  without  calling  for  the  smallest 
trouble  in  relighting  them,  which  cannot  be  done  in  the  case 
of  gas.  In  effecting  a  comparison  of  cost,  we  have  to  take 
into  consideration  the  time  of  wastage  and  non-useful  com- 
bustion in  the  consumption  of  gas ;  and  this,  apart  altogether 
from  the  immense  superiority  of  the  illuminant  which  fouls 
no  air  and  destroys  no  decorations.  Actual  extended  ex- 
perience shows*  that,  with  care  and  good  management, 
electric  energy  at  8d.  per  unit  can  be  made  to  give  as  much 
illumination  by  means  of  incandescent  lamps  as  gas  at  3s.  2d. 
per  thousand  cubic  feet  used  with  ordinary  gas  burners.  At 
this  rate,  one  Board  of  Trade  unit  of  electric  energy  can  be 
made  to  do  the  work  of  about  200  cubic  feet  of  gas. 

A  very  important  factor  which  enters  into  the  question 
of  the  cost  of  incandescent  lighting  is  the  care  with  which 
the  arrangements  of  the  electric  light  "  wiring "  have 

*  See  The  Electrician,  March  16,  1894,  p.  561. 


91      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

been  carried  out.  The  "  wiring  "  may  be  done  in  such  a 
way  that  the  user  can  only  turn  on  and  off  large  groups  of 
lamps  at  once.  As  far  as  possible  each  lamp  should  be  on 
a  separate  switch,  and  be  capable  of  being  used  and  put  out 
of  use  alone.  Such  independent  wiring  costs  more  per  light 
than  group  lighting,  but  it  enables  the  user  to  create  the 
necessary  illumination  with  a  minimum  of  expenditure  of 
electric  energy. 

This  is  the  moment  to  offer  a  word  of  caution  against  all 
and  every  form  of  "  cheap  "  electric  lighting  work.  Too  much 
care  cannot  possibly  be  bestowed  upon  the  quality  of  the 
materials  and  character  of  the  work  in  laying  the  electric  service 
lines  and  mains  in  buildings.  This  "  wiring  "  has  now  become 
a  special  trade,  and  in  the  hands  of  competent  firms  is  carried 
out  with  the  utmost  care  and  with  all  the  numerous  pre- 
cautions which  long  experience  has  shown  to  be  necessary 
for  safety  and  efficiency.  No  amount  of  inspection  after  the 
work  is  finished  and  no  amount  of  electric  testing  is  really  an 
efficient  substitute  for  a  watchful  and  experienced  eye  during 
the  progress  of  the  work.  All  important  electric  wiring  work 
ought  to  be  thus  watched  on  behalf  of  the  owners  by  a  clerk 
of  the  works  trained  in  this  kind  of  supervision.  It  is  not 
sufficient  for  a  consulting  engineer  to  draft  the  specification 
of  the  work ;  it  must  be  suitably  supervised  during  its  pro- 
gress. It  is  advisable  that  every  architect  should  thus  be 
competent  to  draft  his  own  specification  for  electric  house- 
wiring,  and  that  every  clerk  of  the  works  should  be  competent 
to  supervise  and  inspect  the  electric  wiring  work  during  its 
entire  progress. 

The  actual  average  annual  consumption  of  electric  energy  by 
incandescent  lamps  placed  in  buildings  of  various  classes  differs 
very  greatly.  Taking  the  case  of  supply  by  meter,  it  is  found 
that  for  a  large  class  of  private  houses  the  average  number  of 


ELECTRIC  GLOW  LAMPS.  95 

Board  of  Trade  units  taken  per  annum  by  an  8-c.p.  lamp  will 
•not  exceed  12  to  20.    .Indeed  20  is  generally  an  outside  limit. 
Lamps  in  clubs,  hotels  and  restaurants  show  a  very  much 
larger  average  than  20,  and  may  reach  30  to  40  B.T.U.  per 
8-c.p.  lamp  per  annum,  or  occasionally  even  more.     It  may 
be  mentioned  that  a  16-c.p.  lamp  would,  under  the  same  condi- 
tions, take  twice,  and  a  32-c.p.  three  times  the  above  amount. 
Since  a  35 -watt  8-c.p.  lamp  takes   one   B.T.U.   of  electric 
energy   in    30    hours  of   continuous    use,    it   follows    that 
from  860  to  600  hours  is   the   average  total  time  during 
which  a  lamp  in  a  private   house  is  in   use  in  the  year. 
That  is,  from  one  hour  to  one  hour  and  a  half  is  the 
average   daily   time   during  which   each  lamp  is  alight  in 
residential   buildings.     During   the  other  23   or  22-J   hours 
of   the   day,   however,   the   electric   supply   station    has   to 
keep  in  readiness,  machinery  or  other  apparatus  capable  of 
giving  current  for  that  lamp  should  the  user  require  it.     The 
load  factor  of  a  supply   station  is  the   ratio  of  the   actual 
amount  of  Board  of  Trade  units  sold  to  the  amount  which 
could  be  sold  if  the  demand  were  constant  and  equal  to  the 
maximum  demand.     It  will  be  seen  from  the  above  figures 
that  the  load  factor  of  an  electric  supply  station,  supplying  a 
purely  residential  district,   does    not    exceed   8   or   10  per 
cent.     The  load  factors  of  various  classes   of  buildings  are 
very  different.     The  load  factor  of  private  houses  is  seen 
from  the  above  figures  to  be  even  below  8  per  cent.     The 
most  profitable  consumers,  therefore,  from  the  point  of  view 
of  the  suppliers  of  electric  current,  are  not  those  who  have 
most  lamps,  but  those  who  use  the  lamps  they  have  for  the 
longest  number  of  hours. 

"We  will  now  turn  to  consider  the  manner  in  which 
incandescent  lamps  can  be  employed  in  order  to  produce 
the  best  illuminating  effect.  The  immense  advantage 
which  electric  incandescent  lighting  possesses  is,  that  it 


96      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

lends  itself  so  readily  to  decorative  purposes.  In  order  that 
it  may  be  employed  in  this  sense  to  the  best  advantage, 
certain  general  guiding  principles  have  to  be  held  in  view 
and  followed.  Unfortunately,  this  is  not  always  done,  and  the 
result  is  that  the  effectiveness  of  the  light,  from  the  artistic 
point  of  view,  is  greatly  diminished. 

The  first  .principle  which  should  be  allowed  to  guide  us  in 
the  production  of  an  artistic  effect  by  electric  lighting  is  that, 
while  a  proper  and  sufficient  illumination  is  thrown  upon 
the  surfaces  to  be  illuminated,  the  source  of  light  must  be 
itself,  as  far  as  possible,  concealed.     The  illuminating  power 
of  a  light  for  the  purpose  of  vision  does  not  depend  merely 
upon  the  candle-power  of  the  lamp,  but  it  depends  upon  the 
amount  of  light  which  is  received  by  the  eye  from  the  surfaces 
from  which  it  is  reflected.     The  pupil  of  the  eye  is  capable  of 
being  varied  over  wide  limits,  probably  from  about  four  square 
millimetres,  or  the  150th  part-  of  a  square  inch,  to  70  square 
millimetres  or  the  tenth  part  of  a  square  inch.      This  ex- 
pansion or  contraction  of  the  pupil  of  the  eye  is  effected  by 
muscles  over  which  the  will  has  no  control,  and  is  termed 
in  physiology  a  "  reflex  action."     If  the  eye  has  been  kept  in 
darkness  for  some  time,  then  the  pupil  becomes  expanded  to 
its  greatest  extent.      Under  these  conditions,  if  the  eye  is 
opened,  and  especially  if  it  is  exposed  to  a  bright  light,  this  light 
falling  on  the  retina  causes   an  immediate  muscular  con- 
traction of  the  pupil  to  be  effected.     It  is  very  easy  to  see  this 
adjusting  process  going  on  in  the  eye  by  shutting  the  eyes  for 
a  few  minutes  and  then  opening  them  in  a  bright  light  whilst 
a  hand  mirror  is  held  before  the  face  so  as  to  examine  the 
pupils  of  the  eyes.     The  pupils  will  then  be  seen  to  be  rapidly 
contracting  in  area  immediately  the  eyes  are  opened.     Hence, 
if  the  eye  is  turned  upon  a  brilliant  line  of  light,  the  contraction 
of  the  pupil  which  immediately  sets  in  imposes  a  limitation 
upon  the  amount  of  light  which  can  enter  the  eye ;  and  if  the 


ELECTRIC  GLOW  LAMPS.  9? 

eye  is  turned  immediately  towards  a  dull,  badly  illuminated 
surface,  the  pupil  is  not  at  once  suddenly  expanded  again. 

One  part,  at  any  rate,  of  the  injurious  effect  produced  by 
trying  to  read  or  write  in  the  twilight  is  due  to  the  strain 
produced  in  the  eyes  by  the  muscular  effort  thus  demanded  in 
the  eye.  When,  therefore,  we  enter  a  room  in  which  there 
are  a  number  of  bright  lights,  such,  for  instance,  as  a  room  in 
which  incandescent  lamps  are  being  employed,  the  filaments 
of  which  can  be  seen,  the  moment  that  the  eye  is  turned  upon 
these  bright  lines  of  light,  muscular  contraction  of  the  pupil  sets 
in,  and  on  turning  the  eye  away  again  to  other  less  illuminated 
surfaces,  the  retina  of  the  eye  does  not  receive  sufficient  light 
to  observe  details.  In  common  language,  the  eye  is  "  dazzled." 
There  is  a  certain  fascination  about  brilliant  points  or  lines  of 
light  which  captivates  and  attracts  the  eye  even  against  the 
will. 

It  will  thus  be  seen  that  the  presence  of  brilliant  lines 
or  points  of  light  in  a  room  has  a  tendency  to  keep  the  pupil 
of  the  eye  in  a  state  of  partial  contraction,  which  unfits  it  for 
obtaining  the  best  visual  effect  when  turned  upon  surfaces  less 
illuminated.  Some  eyes  are  especially  sensitive  in  this  respect. 
If,  however,  the  lamp  globes  are  made  of  frosted  glass,  or  in 
other  ways  protected,  so  that  the  image  of  the  incandescent 
filament  cannot  be  directly  thrown  upon  the  retina,  the  actual 
visual  effect  may  be  increased  in  spite  of  the  fact  that  such 
frosted  globes  or  screens  may  cut  off  from  30  to  50  per  cent,  of 
the  light.  This  is  a  common  experience  with  everybody  who 
tries  to  read  or  write  in  the  neighbourhood  of  a  brilliant  in- 
candescent lamp.  By  covering  the  globe  with  tissue  paper,  or 
with  a  ground  glass  or  porcelain  shade,  although  a  diminution 
to  a  large  extent  is  caused  in  the  total  light  actually  dis- 
tributed from  the  lamp,  yet,  nevertheless,  we  are  able  to  see 
better  and  more  comfortably  by  it.  The  process,  however, 


9$      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

which  is  commonly  adopted  of  cutting  off  a  large  proportion 
of  the  light  by  a  semi- opaque  globe  is  a  wasteful  one,  because 
there  can  be  no  object  ?ji  making  light  merely  to  absorb  it. 

The  proper  method  is  to  so  place  the  incandescent  lamp 
that  no  light  from  the  filament  can  directly  enter  the  eye, 
but  so  that  all  the  light  sent  out  from  the  filament  shall  be 
reflected  to  the  eye  from  the  objects  to  be  seen,  such  as 
the  walls  of  the  room  and  the  various  objects  in  the  room, 


'   FIG.  36. — View  of  the  Interior  of  a  Dining  Room,  Illuminated  with 
Shaded  Electric  Candles. 

and  that  the  light  shall  reach  the  eye  only  after  reflection 
from  these  surfaces.  Bare  or  uncovered  lamps,  and  especially 
if  suspended  half  way  down  a  room  or  on  a  level  with,  or 
a  little  above  the  eye,  are  a  crude  and  exceedingly  disagreeable 
method  of  illumination. 

,.'      ,      ,     '  "          •':*•.••.          (     -  *•<••'.,•  ">       •    "       •''•••-    '. i    •    •     •  •.,.(• 

In  all  cases  where  incandescent  lighting  is  carried  out  by  those 
who  understand  the  proper  methods  of  using  it,  so  as  to  pro- 


ELECTRIC  GLOW  LAMPS.  90 

duce  the  best  artistic  effects,  the  guiding  principle  is  followed 
of  so  placing  the  lamps  that  the  whole  of  the  light  emitted 
from  them  only  reaches  the  eye  after  reflection  from  sur- 
rounding objects.  This  result  can  be  attained  in  a  variety 
of  ways.  The  lamps  may  be  placed  in  shades  so  as  to 
throw  the  light  upwards  upon  the  ceiling  of  the  room  or 
upon  the  walls,  from  which  surfaces  it  is  ultimately  reflected 
down  on  to  the  objects  in  the  room.  Wherever  highly 
decorated  and  light  ceilings  exist  in  a  room  to  be  lit,  this  is 
a  very  effective  method  of  distributing  the  light.  In  Figs.  36, 
37,  38,  39,  and  40,  are  shown  photographs  of  interiors  so 
illuminated.  It  is  always  possible  to  arrange  glow  lamps 
protecting  them  by  metallic  or  other  ornamental  shields,  so 
so  as  to  obtain  this  desired  effect.  A  natural  shell,  properly 
supported,  forms  a  very  effective  screen  and  reflecting  surface. 
The  white  pearly  interior  of  the  shell  acts  as  a  reflecting 
surface,  and  if  the  shell  is  very  slightly  translucent,  then 
a  pleasing  effect  is  produced  by  the  small  and  diffused  light 
which  passes  through. 


The  second  important  principle  of  decorative  electric 
lighting  is  to  distribute  the  light  properly  and  to  prevent 
the  concentration  of  light  in  large  masses,  thereby  avoid- 
ing the  production  of  harsh  shadows.  Everyone  is  familiar 
with  the  exceedingly  disagreeable  effect  which  is  produced 
by  sharp  and  strong  shadows.  A  soft  gradation  of 
light  and  shade  is  essential  in  the  production  of  an 
artistic  effect  in  a  room.  The  inartistic  productions  of 
many  amateur  photographers,  especially  in  the  region  of 
portraiture,  are  much  more  due  to  the  fact  that  they  have 
not  the  means  at  their  disposal  for  producing  the  right 
effects  of  light  and  shade  on  their  subjects,  than  to  any 
defects  in  their  photographic  manipulation.  Any  concentra- 
tion of  the  light  in  large  masses,  in  public  or  private  rooms, 

II  2 


100  'ELECTRIC  LAMPS  AND  ELECTRIC 


FIG.  37.— View  of  the  Interior  of  a  Drawing  Room  Lighted  Electrically 
with  Screened  Lamps. 


FIG.  38. — View  of  a  Ball  Room  Illuminated  with  Shaded  Electric 
Candles  in  Chandeliers. 


ELECTRIC  GLOW  LAMPS. 


101 


Fig.  39. — View  of  a  Reception  Room  Illuminated  with  Inverted 
Incandescent  Lamps  throwing  Light  upwards. 


FIG.  40, — View  of  a  Drawing  Room  Illuminated  with  Shaded 
Electric  Candles, 


102    ELECTRIC  LAMPS  AND  ELECTEIC  LIGHTING. 

and  especially  when  thrown  downwards,  invariably  produces 
disagreeable  shadows  upon  the  face;  and,  therefore,  in  places 
like  ball-rooms  or  drawing-rooms  no  such  process  of  employ- 
ing downwardly-suspended  incandescent  lamps  should  ever  be 
permitted  if  it  is  desired  to  obtain  the  best  results.  All  artists 
know  the  immense  importance  of  obtaining  proper  lighting 
by  diffused  light,  and  the  northern  aspect  which  they  invariably 
select  for  their  studios  is  determined  by  this  consideration. 
It  is  essential  that  the  light  should  be  so  distributed  in  small 
units,  in  order  that  there  shall  be  no  sharp  shadows  cast 
upon  any  object.  A  simple  test  for  the  effectiveness  of  the 
distribution  of  light  in  any  room  may  be  obtained  as  follows : 
Take  a  white  card  or  a  sheet  of  white  writing  paper  and  hold 
it  horizontally  about  the  level  of  the  eyes,  then  hold  a  pencil 
or  other  small  rod  vertically  on  the  card,  and  note  if  any 
marked  shadow  of  the  pencil  is  thrown  in  any  direction :  if  it 
is,  then  the  light  is  insufficiently  diffused,  and  the  lamps 
ought  to  be  re-arranged.  The  best  effects  as  to  distribution 
of  light  are  generally  obtained  by  the  employment  of  5  candle- 
power  lamps.  These  glow  lamps  are  constructed  with  bulbs 
of  semi-opaque  glass  having  the  shape  of  a  candle  flame. 
(see  Fig.  21).  These  candle  lamps  are  carried  on  the  ex- 
tremity of  an  artificial  porcelain  candle,  so  as  to  resemble  in 
some  degree  an  ordinary  wax  candle  alight.  These  lamps  are 
then  arranged  in  groups  on  chandeliers  or  brackets,  being  pro- 
tected in  many  cases  from  direct  visibility  by  plain  or  orna- 
mental shades.  In  Figs.  36  and  41  are  shown  photographic 
illustrations  of  rooms  which  are  lit  by  such  candle  lamps,  and 
in  which  the  light  from  these  is  thrown  up  against  the  walls 
of  the  room,  the  candle  lamps  not  being  themselves  directly 
visible. 

A  third  great  guiding  principle  of  artistic  lighting  is  to  pro- 
portion the  light  to  the  nature  of  the  surfaces  from  which  it 
is  reflected.  We  require  light  in  our  living  rooms  to  see  the 


ELECTEIC  GLOW  LAMPS. 


103 


FIG.  41. — Drawing  Room  Illuminated  with  Electric  Lamps,  so  shaded  ; 
that  no  direct  view  of  the  lamp  filament  can  be  obtained. 


Fia.  42. — Interior  of  Freemason's  Hall  Illuminated  by  Shaded  Incandescent 
Lamps  on  Frieze  and  Ceiling. 


104     ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


pictures,  decorations,  objects,  and,  above  all,  to  see  one  another; 
and  we  can  only  see  these  objects  by  the  light  which  they 
reflect  back  to  the  eye.  The  aim  should,  therefore,  be  to  have 
sufficient  but  not  excessive  incident  light.  It  should  be  borne 
in  mind  that  the  different  surfaces  with  which  the  walls  of 
rooms  are  covered  have  very  different  reflective  powers.  Dull 
walls  absorb  eighty  per  cent,  of  the  light  falling  on  them,  and 
reflect  only  about  twenty  per  cent,  of  the  light ;  this  is  the  case, 
for  instance,  with  walls  covered  with  dark  oak  panelling  or  dull- 
coloured  paint  or  paper.  Ordinary  light  tints  of  paper  or 
paint  may  reflect  from  forty  to  sixty  per  cent. ;  clean  white 
surfaces,  such  as  white  painted  and  varnished  wood  or  plaster, 
may  reflect  as  much  as  eighty  per  cent.,  and  mirrors  from 
eighty  to  ninety  per  cent. 

The  following  figures  have  been  given  by  Dr.  Sumpner 
for  the  reflecting  power  of  various  surfaces : — 

Table  of  Reflecting  Powers  of  Various  Surfaces. 


White  blotting  paper  82  per  cent. 

Ordinary  foolscap   ...  70  „ 

Newspapers 50-70  „ 

Yellow  wall  paper  ...  40  „ 

Blue  paper  25  „ 

Dark  brown  paper  ...  13  „ 

Deep  chocolate  paper    4  „ 


Plane  deal  (clean)  40-50  per  cent. 

Plane  deal  (dirty)  ...  20  „      „ 
Yellow  painted  wall 

(dirty)     20  „      „ 

Yellow  painted  wall 

(dean)     40  „      „ 

Black  cloth  1-2  „      „ 

Black  velvet    .          .    *4  , 


Everyone  is  familiar  with  the  way  in  which  the  laying 
of  a  white  table-cloth  brightens  up  a  dark  dining  room,  or 
in  which  the  substitution  of  white  for  dark  curtains  lightens 
up  a  room.  The  reason  for  this  is  obvious  from  the  above 
table.  Hence,  rooms  with  dark  oil  paintings,  dark  woodwork, 
hangings,  curtains,  or  portieres  require  relatively  much  more 
illumination  per  square  foot  than  rooms  with  very  light 
decorations,  water-colour  paintings,  mirrors,  &c.  Experience 
has  shown  that  in  a  room  of  the  latter  description  an  illumi- 


ELECTEIC  GLOW  LAMPS.  105 

nation  is  required  which  is  equal  to  an  expenditure  in  the 
lamp  of  about  f  or  1  watt  per  square  foot  of  floor  surface,  on 
the  assumption  that  the  lamps  are  placed,  on  an  average, 
about  8  or  9  feet  above  the  floor.  A  room  of  the  former 
description,  such  as  a  picture  gallery  with  dark  oak  walls, 
may  require  from  2  to  3  watts  per  square  foot  of  floor  surface. 
These  figures,  however,  cannot  be  taken  too  absolutely. 
Experience  is  the  only  guide  as  to  the  amount  of  light  to  be 
placed  in  a  room.  As  a  broad  general  rule,  100  square  feet 
of  floor  surface  will  be  barely  illuminated  by  one  16-c.p. 
lamp  placed  about  8  feet  above  the  floor.  It  will  be 
well  illuminated  by  two  such  lamps  placed  8  feet  above  the 
floor,  and  brilliantly  illuminated  by  four  such  lamps.  In 
other  words,  one  16-c.p.  lamp  per  100  square  feet  of 
floor  surface  is  hardly  sufficient,  one  16-c.p.  lamp  per  50 
square  feet  of  floor  surface  is  fairly  good,  and  one  for  every 
25  square  feet  of  floor  surface  is  brilliant.  These  figures 
are  given  on  the  assumption  that  the  lamp  is  placed  about 
8  feet  above  the  floor,  and  that  the  floor  surface  is  fairly 
reflective.  The  lighting  of  pictures  is  a  very  special  study,  and 
can  only  be  properly  undertaken  by  those  who  have  given 
considerable  attention  to  the  method  of  placing  and  using 
incandescent  lamps.  The  mere  illumination  of  a  room  by  a 
number  of  lamps  hung  head  downwards  by  cord  pendants 
from  the  ceiling,  a  method  which  is  not  unusually  adopted,  is 
the  most  crude  and  imperfect  method  of  applying  the  electric 
light,  and  ought  never  to  be  permitted  in  any  case  where  the 
smallest  decorative  effect  is  really  desired. 

The  brief  limits  of  a  lecture  will  not  permit  us  to  enlarge 
at  greater  length  on  this  part  of  the  subject,  but  it  opens  up 
a  wide  field  for  the  exercise  of  the  inventive  and  aesthetic 
faculties. 

Before  leaving  the  subject  of  incandescent  lamps  it  will 
not  be  without  interest  to  refer  briefly  to  the  investigations 


100    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

which  have  been  made  into  a  curious  effect,  found  to  exist 
in  lamps  which  have  a  third  or  idle  metallic  wire  i  sealed 
through  the  glass.  Our  starting-point  for  this  purpose  is  a 
discovery  made  by  Mr.  Edison  in  1884,  and  which  received 
examination  at  the  hands  of  Sir  W.  H.  Preece  in  the  follow- 
ing year,*  and  by  the  author  more  recently.  Here  is  the 
initial  experiment.  A  glow-lamp  having  the  usual  horseshoe- 
shaped  carbon  (Fig.  43)  has, a  metal  plate  held  on  a  platinum 
wire  sealed  through  the  glass  bulb.  This  plate  is  so  fixed  that 
it  stands  up  between  the  two  sides  of  the  carbon  arch  without 
touching  either  of  them.  We  shall  illuminate  the  lamp  by  a! 


Fia.  43. — Glow  Lamp  having  insulated  metal  middle  plate  M  sealed  into 
the  bulb  to  exhibit  the  "  Edison  effect." 

continuous  current  of  electricity,  and  for  brevity's  sake  speak 
of  that  half  of  the  loop  of  carbon  on  the  side  by  which  the 
current  enters  it  as  the  positive  leg,  and  the  other  half  of  the 

*  Sir  W.  H.  Preece's  interesting  Paper  on  this  subject  is  published  in  the 
Proceedings  of  the  Royal  Society  for  1885,  p.  219.  See  also  The  Electrician, 
April  4,  1885,  p.  436,  "  On  a  Peculiar  Behaviour  of  Glow-lamps  when 
Raised  to.  High  .Incandescence."  .The  following,  statements  of  experiments 
are  chiefly  taken  from  a  Friday  evening  discourse  given  by  the  author  at 
the  Royal  Institution,  February  14,  1890,  on  "  Problems  in  the  Physics  of 
an  Electric  Lamp,"  and  from  a  Paper  on  "Electric  Discharge  between 
Electrodes  at  different  Temperatures  in  Air  and  in  High  Vacua."  (Proceed- 
ings of  ike  Royal  Society-,  - 1890. ) 


ELECTRIC  GLOW  LAMPS. 


107 


loop  as  the  negative  leg.  The  diagram  in  Fig.  45  shows  the 
position  of  the  plate  with  respect  to  the  carbon  loop.  There 
is  a  distance  of  half-an-inch,  or  in  some  cases  many  inches, 
between  either  leg  of  the  carbon  and  this  middle  plate. 
Setting. the  lamp  in  action,  I  connect  a  sensitive  galvanometer 
between  the  middle  plate  and  the  negative  terminal  of  the 
lamp,  and  you  see  that  there  is  no  current  passing  through 
the  instrument  If,  however,  I  connect  the  terminals  of  my 
galvanometer,  to  the  middle  plate,  and  to  the .  positive. -electrode 
of, the  lamp,  we;find  a  current  of  some  milliamperes  is  passing 
through  it.  TM>  diagrams  in  Fig.  45  show  the  mode  -of 


FIG.  44. — Sensitive  Galvanometer  connected  between  the  middle  plate 
and  positive  electrode  of  a  Glow  Lamp,  showing  current  flowing  through  it 
when  the  lamp  is  in  action  ("  Edison  effect "). 

nection  of  the  galvanometer  in  the  two  cases.  This  effect, 
which  is  often  spoken  of  as  the  "Edison  effect,"  clearly, 
indicates  that  an  insulated  plate  so  placed  in  the  vacuum  of 
a  lamp  in  action  is  brought  down  to  the  same  potential  or 
electrical  state  as  the  negative  electrode  of  the  carbon  loop. 
On  examining  the  direction  of  the  current  through  the  galva- 
nometer we  find  that  it  is  equivalent  to  a  flow  of  negative 
electricity  taking  place  through  it  from  the  middle plate  to  the. 


108    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

positive  electrode  of  the  lamp.  A  consideration  cf  this  fact 
shows  us  that  there  must  be  some  way  by  which  negative 
electricity  gets  across  the  vacuous  space  from  the  negative  leg 
of  the  carbon  to  the  metal  plate,  whilst  at  the  same  time  a 
negative  charge  cannot  pass  from  the  metal  plate  across  to 
the  positive  leg. 

Before  passing  away  from  this  initial  experiment,  the  atten- 
tion of  the  reader  should  be  called  to  a  curious  effect  at  the 
moment  when  the  lamp  is  extinguished.  Connecting  the 
galvanometer,  as  at  first,  between  the  middle  plate  and  the 
negative  electrode  of  the  lamp,  we  notice  that,  though  made 


(No  current.)  (A  current.) 

FIG.  45. — Mode  of  connection  of  galvanometer  G  to  middle  plate  J/and 
carbon  horseshoe-shaped  conductor  C  in  the  experiment  of  the  "  Edison 
effect." 


highly  sensitive,  the  galvanometer  indicates  no  current  flowing 
through  it  whilst  the  lamp  is  in  action.  Switching  off  the 
current  from  the  lamp  produces,  as  you  see,  a  violent  kick  or 
deflection  of  the  galvanometer,  indicating  a  sudden  rush  of 
current  through  it. 

In  endeavouring  to  ascertain  further  facts  about  this  effect 
one  of  the  experiments  which  early  suggested  itself  was 
directed  to  determine  the  relative  effects  of  different  portions  of 
the  carbon  conductor.  Here  is  a  lamp  (see  Fig  46)  in  which 


ELECTRIC  GLOW  LAMPS. 


109 


one  leg  of  the  carbon  horseshoe  has  been  enclosed  in  a  glass 
tube  the  size  of  a  quill,  which  shuts  in  one  half  of  the  carbon. 
The  bulb  contains,  as  before,  an  insulated  middle  plate.  If  we 
pass  the  actuating  current  through  this  lamp  in  such  a  direc- 
tion that  the  covered  or  sheathed  leg  is  the  positica  leg,  we 
find  the  effect  existing  as  before.  A  galvanometer  connected 
between  the  plate  and  positive  terminal  of  the  lamp  yields  a 
strong  current,  whilst  if  connected  between  the  negative 
terminal  and  the  middle  plate  there  is  no  current  at  all.  Let 
us,  however,  reverse  the  current  through  the  lamp  so  that  the 


FIG.  46.— Glow  Lamp  having  negative  leg  of  carbon  enclosed  in  glass  tubo 
T,  the  "  Edison  effect"  being  thereby  annul  led  or  greatly  diminished. 


shielded  or  enclosed  leg  is  now  the  negative  one,  and  the  galva- 
nometer is  able  to  detect  no  current,  whether  connected  in  one 
way  or  in  the  other.  We  establish,  therefore,  the  conclusion 
that  it  is  the  negative  leg  of  the  carbon  loop  which  is  the  active 
agent  in  the  production  of  this  "Edison  effect,"  and  that,  if 
it  is  enclosed  in  a  tube  of  either  glass  or  metal,  no  current 
is  found  flowing  in  a  galvanometer  connected  between  the 
positive  terminal  of  the  lamp  and  this  middle  collecting  plate. 


110    ELECTEIO  LAMPS  AND  ELECTEIO  LIGHTING. 

Another  experiment  which  confirms  this  view  is  as  follows : 
The  lamp  (Fig.  47)  has  a  middle  plate,  which  is  provided 
with  a  little  mica  flap  or  shutter  on  one  side  of  it.  When  the 
lamp  is  held  upright  the  mica  shield  falls  over  and  covers  one 
side  of  the  plate,  but  when  it  is  held  in  a  horizontal  position 
the  mica  shield  falls  away  from  the  front  of  the  plate  and 
exposes  it.  Using  this  lamp  as  before,  we  find  that,  when  the 
positive  leg  of  the  carbon  loop  is  opposite  to  the  shielded  face 
of  the  plate,  we  get  the  "  Edison  effect "  as  before  in  any  posi- 
tion of  the  lamp.  Reversing  the  lamp  current,  and  making 


FIG.  47.  -Glow  Lamp  having  mica  shield  S  interposable  between  middle 
plate  M  and  negative  leg  of  carbon,  thereby  diminishing  the  "Edison 
effect." 

that  same  leg  the  negative  one,  we  find  that,  when  the  lamp  is 
so  held,  the  metal  plate  is  shielded  by  the  interposition  of  the 
mica,  and  the  galvanometer  current  is  very  much  less  than 
when  the  shield  is  shaken  on  one  side  and  the  plate  exposed 
fully  to  the  negative  leg 

At  this  stage  it  will  perhaps  be  most  convenient  to  outline 
briefly  the  beginnings  of  a  theory  which  may  be  proposed  to 
reconcile  these  facts,  and  leave  .you  to  judge  how  far  the 


ELECTRIC  GLOW  LAMPS.  Ill 

subsequent  experiments  seem  to  confirm  this  hypothesis. 
Very  briefly,  this  theory  is  as  follows :  From  all  parts  of  the 
incandescent  carbon  loop,  but  chiefly  from  the  negative  leg, 
carbon  molecules  are  being  projected  which  carry  with  them, 
or  are  charged  with,  negative  electricity.  In  a  few  moments 
a  suggestion  will  be  made  to  you  which  may  point  to  a 
possible  hypothesis  on  the  manner  in  which  the  molecules 
acquire  this  negative  charge.  Supposing  this,  however,  to  be 
the  case,  and  that  the  bulb  is  filled  with  these  negatively- 
charged  molecules,  what  would  be  the  result  of  introducing 
into  their  midst  a  conductor  such  as  this  middle  metal  plate 
which  is  charged  positively  ?  Obviously,  they  would  all  be 
attracted  to  it  and  discharge  against  it.  Suppose  the  positive 
charge  of  this  conductor  to  be  continually  renewed,  and  the 
negatively-charged  molecules  continually  supplied — which 
conditions  can  be  obtained  by  connecting  the  middle  plate  to 
the  positive  electrode  of  the  lamp — the  obvious  result  will 
be  to  produce  a  current  of  electricity  flowing  through  the  wire 
or  galvanometer  by  means  of  which  this  middle  plate  is 
connected  to  the  positive  electrode  of  the  lamp.  If,  however, 
the  middle  plate  is  connected  to  the  negative  electrode  of  the 
lamp,  the  negatively-charged  molecules  can  give  up  no  charge 
to  it,  and  produce  no  current  in  the  interpolated  galvanometer. 
We  see  that,  on  this  assumption,  the  effect  must  necessarily  be 
diminished  by  any  arrangement  which  prevents  these  nega- 
tively-charged molecules  from  being  shot  off  the  negative  leg 
or  from  striking  against  the  middle  plate. 

Another  obvious  corollary  from  this  theory  is  that  the 
"Edison  effect"  should  be  annihilated  if  the  metal  collect- 
ing plate  is  placed  at  a  distance  from  the  negative  leg  much 
greater  than  the  mean  free  path  of  the  molecules. 

Here  are  some  experiments  which  confirm  this  deduction. 
In  this  bulb  (Fig.  48)  the  metal  collecting  plate,  which  is  to  be 


112    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

connected  through  the  galvanometer  with  the  positive  terminal 
of  the  lamp,  is  placed  at  the  end  of  a  long  tube  opening  out 
of  and  forming  part  of  the  bulb.  We  find  the  "  Edison 
effect "  is  entirely  absent,  and  that  the  galvanometer  current 
is  zero.  We  have,  as  it  were,  placed  our  target  at  such  a 
distance  that  the  lowest  range  molecular  bullets  cannot  hit 
it,  or  at  least  that  but  very  few  of  them  do  so.  Here,  again,  is 
a  lamp  in  which  the  plate  is  placed  at  the  extremity  of  a  tube 
opening  out  of  the  bulb,  but  bent  at  right  angles  (Fig.  49). 
We  find  in  this  case,  as  first  discovered  by  Mr.  W.  H.  Preece, 
that  there  is  no  "  Edison  effect."  Our  molecular  marksman 


FIG.  48.— Collecting  plate  placed  at  end  of  tube,  18in.  in  length,  opening 
out  of  bulb. 

cannot  uhoot  round  a  corner.  None  of  the  negatively-charged 
molecules  can  reach  the  plate,  although  that  plate  is  placed 
at  a  distance  not  greater  than  would  suffice  to  produce  the 
effect  if  the  bend  were  straightened  out.  Following  out  our 
hypothesis  into  its  consequences  would  lead  us  to  conclude 
that  the  material  of  which  the  plate  is  made  is  without 
influence  on  the  result,  and  this  is  found  to  be  the  case. 

We  should  expect  also  to  find  that  the  larger  we  make  our 
plate,  and  the  nearer  we  bring  it  to  the  negative  leg  of  the 
carbon,  the  greater  will  be  the  current  produced  in  a  circuit 


ELECTRIC  GLOW  LAMPS. 


113 


connecting  this  plate  to  the  positive  terminal  of  the  lamp.  I 
have  before  me  a  lamp  with  a  large  plate  placed  very  near  the 
negative  leg  of  the  carbon  of  a  lamp,  and  we  find  that  we 
can  collect  enough  current  from  these  molecular  charges  to 
Tork  a  telegraph  relay  and  ring  an  electric  bell.  The  current 
which  is  now  working  this  relay  is  made  up  of  the  charges 
collected  by  the  plate  from  the  negatively-charged  carbon 
molecules,  which  are  projected  against  it  from  the  negative  leg, 
across  the  highly  perfect  vacuum.  I  have  tried  experiments 
with  lamps  in  which  the  collecting  plate  is  placed  in  all  kinds 


FIG.  49. — Collecting  plate  placed  at  end  of  elbow  tube  opening  out  of  bulb. 

of  positions,  but  the  results  may  all  be  summed  up  by  saying 
that  the  greatest  effects  are  produced  when  the  collecting  plate 
is  as  near  as  possible  to  the  base  of  the  negative  end  of  the 
loops,  and,  as  far  as  possible,  encloses,  without  touching,  the 
carbon  conductor.  It  is  not  necessary  to  make  more  than 
a  passing  reference  to  the  fact  that  the  magnitude  of  the 
current  flowing  through  the  galvanometer  when  connected 
between  the  middle  plate  and  the  positive  terminal  of  the  lamp 
often  "  jumps "  from  a  low  to  a  high  value,  or  vice  versa, 
in  a  remarkable  manner,  and  that  this  sudden  change  in  the 
current  can  be  produced  by  bringing  strong  magnets  near  the 
outside  of  the  bulb, 


114    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

Let  us  now  follow  out  into  some  other  consequences  the 
hypothesis  that  the  interior  of  the  bulb  of  a  glow  lamp,  when 
in  action,  is  populated  by  flying  crowds  of  carbon  atoms  all 
carrying  a  negative  charge  of  electricity.  Suppose  we  connect 
our  middle  collecting  plate  with  some  external  reservoir  of 
electric  energy,  such  as  a  Ley  den  jar,  or  with  a  condenser 
equivalent  in  capacity  to  many  hundreds  of  Ley  den  jars, 
and  let  the  side  of  the  condenser  which  is  charged  positively 
be  first  placed  in  connection  through  a  galvanometer  with  the 
middle  plate  (see  Fig.  50),  whilst  the  negative  side  is  placed 


FIG.  50. — Charged  condenser  0  discharged  by  middle  plate  M,  when  the 
positively -charged  side  of  condenser  is  in  connection  with  the  plate  and 
the  other  side  to  earth  e. 


in  connection  with  the  earth.  Here  is  a  condenser  of  two 
microfarads  capacity  so  charged  and  connected.  Note  what 
happens  when  I  complete  the  circuit,  and  illuminate  the  lamp 
by  passing  the  current  through  its  filament.  The  condenser 
is  at  once  discharged.  If,  however,  we  repeat  the  same  ex- 
periment, with  the  sole  difference  that  the  negatively- charged 
side  of  the  condenser  is  in  connection  with  the  middle  plate; 
then  there  is  no  discharge. 


ELECTBIC  GLOW  LAMPS.  115 

These  very  interesting  experimental  results  may  also 
be  regarded  from  another  point  of  view.  In  order  that  the 
condenser  may  be  discharged,  as  in  the  first  case,  it  is  essential 
that  the  negatively- charged  side  of  the  condenser  shall  be  in 
connection  with  some  part  of  the  circuit  of  the  incandescent 
carbon  loop.  This  experiment  with  the  condenser  discharged 
by  the  lamp  may  be,  then,  looked  upon  as  an  arrangement  in 
which  the  plates  of  a  charged  condenser  are  connected  respec- 
tively to  an  incandescent  carbon  loop  and  to  a  cool  metal 
plate,  both  being  enclosed  in  a  highly-vacuous  space ;  and  it 
appears  that  the  discharge  takes  place  when  the  incandescent 
conductor  is  the  negative  electrode  of  this  arrangement,  but 
not  when  the  cooler  metal  plate  is  the  negative  electrode  of 
the  charged  condenser.  The  negative  charge  of  the  condenser 
can  be  carried  across  the  vacuous  space  from  the  hot  carbon 
to  the  colder  metal  plate,  but  not  in  the  reverse  direction. 

This  experimental  result  led  me  to  examine  the  condition 
of  the  vacuous  space  between  the  middle  metal-plate  and  the 
negative  leg  of  the  carbon  loop  in  the  case  of  the  lamp 
employed  in  our  first  experiment.  Let  us  return  for  a 
moment  to  that  lamp.  I  join  the  galvanometer  between  the 
middle  plate  and  the  negative  terminal  of  the  lamp,  and  find, 
as  before,  no  indication  of  a  current.  The  metal  plate  and 
the  negative  terminal  of  the  lamp  are  at  the  same  electrical 
potential.  In  the  circuit  of  the  galvanometer  we  will  insert  a 
single  galvanic  cell,  having  an  electromotive  force  of  rather 
over  one  volt.  In  the  first  place  let  that  cell  be  so  inserted 
that  its  negative  pole  is  in  connection  with  the  middle  plate, 
and  its  positive  pole  in  connection  through  the  galvanometer 
with  the  negative  terminal  of  the  lamp  (see  Fig.  51).  Regard- 
ing the  circuit  of  that  cell  alone,  we  find  that  it  consists  of 
the  cell  itself,  the  galvanometer  wire,  and  that  half-inch  of 
highly  vacuous  space  between  the  hot  carbon  conductor  and 
the  middle  plate,  In  that  circuit  the  cell  cannot  send  any 


116    ELECTRIC  LAMPS  AND  ELECTEIC  LIGHTING. 

sensible  current  at  all,  as  it  is  at  the  present  moment  con- 
nected up.  But  if  we  reverse  the  direction  of  the  cell,  so  that 
its  positive  pole  is  in  connection  with  the  middle  plate,  the 
galvanometer  at  once  gives  indications  of  a  very  sensible 
current.  This  highly-vacuous  space,  lying  between  the 
middle  metal  plate  on  the  one  hand  and  the  incandescent 
carbon  on  the  other,  possesses  a  kind  of  unilateral  conJuc- 
tivity,  in  that  it  will  allow  the  current  from  a  single  galvanic 
cell  to  pass  one  way  but  not  the  other.  It  is  a  very  old  and 
familiar  fact,  that  in  order  to  send  a  current  from  a  battery 
through  a  highly-rarefied  gas  by  means  of  metal  electrodes 


Fia.  51. — Current  from  Clark  cell  Ck  being  sent  across  vacuous  space 
between  negative  leg  of  carbon  and  middle  plate  M.  Positive  pole  of  cell 
in  connection  with  plate  M  through  galvanometer  G. 

the  electromotive  force  of  the  battery  must  exceed  a  certain 
value.  Here,  however,  we  have  indication  that  if  the  negative 
electrode  by  which  that  current  seeks  to  enter  the  vacuous 
space  is  made  incandescent  the  current  will  pass  at  a  very 
much  lower  electromotive  force  than  if  the  electrode  is  not 
so  heated. 

A  little  consideration  of  the  foregoing  experiments  led  to  the 
conclusion  that,  in  the  original  experiment,  as  devised  by  Mr. 


ELECTRIC  GLOW  LAMPS. 


117 


Edison,  if  we  could  by  any  means  render  the  middle  plate  very 
hot,  we  should  get  a  current  flowing  through  a  galvanometer 
when  it  is  connected  between  the  middle  plate  and  the  negative 
electrode  of  the  carbon.  This  experiment  can  be  tried  in  the 
manner  now  to  be  shown.  Here  is  a  bulb  (Fig.  52)  having  in  it 
two  carbon  loops :  one  of  these  is  of  ordinary  size,  and  will  be 
rendered  incandescent  by  the  current  from  the  mains.  The 
other  loop  is  very  small,  and  will  be  heated  by  a  well-insulated 
secondary  battery.  This  smaller  incandescent  loop  shall  be 
employed  just  as  if  it  were  a  middle  metal  plate.  It  is,  in 
fact,  simply  an  incandescent  middle  conductor.  On  repeating 


FIG.  52. — Experiment  showing  that  when  the  "middle  plate"  is  a 
carbon  loop  rendered  incandescent  by  insulated  battery  J5,  a  current  of 
negative  electricity  flows  from  M  to  the  positive  leg  of  main  carbon  0 
across  the  vacuum. 

the  typical  experiment  with  this  arrangement,  we  find  that 
the  galvanometer  indicates  a  current  when  connected  between 
the  middle  loop  and  either  the  positive  or  the  negative 
terminal  of  the  main  carbon.  I  have  little  doubt  but  that, 
if  we  could  render  the  platinum  plate  in  our  first-used  lamp 
incandescent  by  concentrating  on  it  from  outside  a  powerful 
beam  of  radiant  heat,  we  should  get  the  same  result. 


118    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

A  similar  set  of  results  can  be  arrived  at  by  experiments 
with  a  bulb  constructed  like  an  ordinary  vacuum  tube,  and 
having  small  carbon  loops  at  each  end  instead  of  the  usual 
platinum  or  aluminium  wires.  Such  a  tube  is  now  before 
you  (see  Fig.  53),  and  will  not  allow  the  current  from  a  few 
cells  of  a  secondary  battery  to  pass  through  it  when  the 
carbon  loops  are  cold.  If,  however,  by  means  of  well- 
insulated  secondary  batteries,  we  render  both  of  the  carbon 
loop  electrodes  highly  incandescent,  a  single  cell  of  a  battery 
is  sufficient  to  pass  a  very  considerable  current  across  that 
vacuous  space,  provided  the  resistance  of  the  rest  of  the 
circuit  is  not  large.  We  may  embrace  the  foregoing  facts 
by  saying  that  if  the  electrodes,  but  especially  the  negative 
electrode,  which  form  the  means  of  ingress  and  egress  of  a 


Fio.  53.— Vacuum  tube  having  carbon  loop  electrodes,  c  c,  at  each  end, 
rendered  incandescent  by  insulated  batteries,  B±  £2,  showing  current  from 
Clark  cell,  Ck,  passing  through  the  high  vacuum  when  the  electrodes  are 
incandescent. 

current  into  a  vacuous  space,  are  capable  of  being  rendered 
highly  incandescent,  and  if  at  that  high  temperature  they 
are  made  to  differ  in  electrical  potential  by  the  application 
of  a  very  small  electromotive  force,  we  may,  under  these  cir- 
cumstances, get  a  very  sensible  current  through  the  rarefied 
gas.  If  the  electrodes  are  cold,  a  very  much  higher 
electromotive  force  will  be  necessary  to  get  the  discharge 
or  current  through  the  space.  These  facts  have  been  made 
the  subject  of  elaborate  investigation  by  Hittorf,  Goldstein, 
and  more  recently  by  J.  J.  Thompson  and  other*. 


ELECTRIC   GLOW  LAMPS.  119 

The  subject  of  electrical  discharge  through  rarefied  air  or 
gases  has  attracted  the  attention  of  many  experimentalists. 
The  outcome  of  their  work  has  been  to  indicate  that  when 
two  electrodes  are  placed  in  a  glass  vessel  containing  a  gas 
under  very  reduced  pressure,  the  action  of  electromotive  force 
is  to  create  two  streams  of  particles  which  may  be  either 
chemical  atoms  or  particles  smaller  than  chemical  atoms 
which  carry  positive  electricity  in  one  direction  and  negative 
in  the  other.  It  may  also  be  that  particles  of  the  electrodes 
are  torn  off  and  take  part  in  this  convective  process.  It  is 
tolerably  certain  that  this  last  effect  does  occur  in  the  case 
of  the  carbon  filament  electric  lamp. 

It  has  been  found  that  a  clean  middle  plate  placed  between 
the  legs  of  the  carbon  horseshoe  becomes  black  soonest  on 
the  side  facing  the  negative  leg  of  the  carbon.  It  has  also 
been  noticed  that,  in  the  case  of  lamps  with  treated  filaments, 
the  surface  of  the  deposit  assumes  a  dead  lamp-black 
appearance  soonest  on  the  negative  leg.  These  observations 
tend  to  support  the  contention  that  there  is  an  ejection,  or 
more  rapid  ejection,  of  carbon  from  the  negative  leg  of  the 
carbon  horseshoe.  Much  has  yet  to  be  done  before  the  full 
meaning  of  this  "  Edison  effect  "  is  unravelled ;  but,  as  far  as 
it  has  yet  been  examined,  the  following  summary  of  facts 
includes  most  of  those  as  yet  observed. 

If  a  platinum  wire  is  sealed  through  the  glass  bulb  of  an 
ordinary  carbon  filament  lamp,  and  carries  at  its  extremity  a 
metal  plate  so  placed  as  to  stand  up  between  the  legs  of  the 
carbon  horseshoe  without  touching  either  of  them,  then,  when 
the  lamp  is  actuated  by  a  continuous  current,  it  is  found 
that : — 

(1.)  This  insulated  metal  plate  is  brought  down  instantly  to 
the  potential  of  the  base  of  the  negative  leg  of  the  carbon,  and 
no  sensible  potential  difference  exists  between  the  insulated 


120    ELECTMIC  LAMPS  AND  ELECT&IC  LIGHTING. 

metal  plate  and  the  negative  electrode  of  the  lamps,  whether 
the  test  be  made  by  a  galvanometer,  by  an  electrostatic  volt- 
meter, or  by  a  condenser. 

(2.)  The  potential  difference  of  the  plate  and  the  positive 
electrode  of  the  lamp  is  exactly  the  same  as  the  working 
potential  difference  of  the  lamp  electrodes,  provided  this  is 
measured  electrostatically,  i.e.,  by  a  condenser,  or  by  an  elec- 
trostatic voltmeter  taking  no  current ;  but,  if  measured  by  a 
galvanometer,  the  potential  difference  of  the  plate  and  the 
positive  electrode  of  the  lamp  is  something  less  than  that  of 
the  working  lamp  electrodes. 

(8.)  This  absolute  equality  of  potential  between  the  negative 
electrode  of  the  lamp  and  the  insulated  plate  only  exists  when 
the  carbon  filament  is  in  a  state  of  vivid  incandescence,  and 
when  the  insulated  plate  is  not  more  than  an  inch  or  so  from 
the  base  of  the  negative  leg.  When  the  lamp  is  at  inter- 
mediate stages  of  incandescence,  or  the  plate  is  considerably 
removed  from  the  base  of  the  negative  leg,  then  the  plate  is 
not  brought  down  quite  to  the  same  potential  as  the  negative 
electrode. 

(4.)  A  galvanometer  connected  between  the  insulated  plate 
and  the  positive  electrode  of  the  lamp  shows  a  current  increas- 
ing from  zero  to  four  or  five  milliamperes,  as  the  carbon 
is  raised  to  its  state  of  commercial  incandescence.  There  is 
not  any  current  greater  than  0-0001  of  a  milliampere*  between 
the  plate  and  the  negative  electrode  when  the  lamp  has  a  good 
vacuum. 

(5.)  If  the  lamp  has  a  bad  vacuum  this  inequality  is 
destroyed,  and  a  sensitive  galvanometer  shows  a  current 
flowing  through  it  when  connected  between  the  middle  plate 
and  either  the  positive  or  negative  electrode. 

(6.)  When  the  lamp  is  actuated  by  an  alternating  current,  a 
continuous  current  is  found  flowing  through  a  galvanometer 
*  A  milliampere  is  the  fuVo^1  °f  an  ampere. 


ELECTKIC  GLOW  LAMPS.  121 

connected  between  the  insulated  plate  and  either  terminal  of 
the  lamp.  The  direction  of  the  current  through  the  galva- 
nometer is  such  as  to  show  that  negative  electricity  is  flowing 
from  the  plate  through  the  galvanometer  to  the  lamp  terminal. 
This  is  also  the  case  in  (4) ;  but,  if  the  lamp  has  a  bad  vacuum, 
then  negative  electricity  flows  from  the  plate  through  the  gal- 
vanometer to  the  positive  terminal  of  the  lamp,  and  negative 
electricity  flows  to  the  plate  through  the  galvanometer  from 
the  negative  terminal  of  the  lamp. 

(7.)  The  same  effects  exist,  on  a  reduced  scale,  when  the 
incandescent  conductor  is  a  platinum  wire  instead  of  a  carbon 
filament.  The  platinum  wire  has  to  be  brought  up  very  near 
to  its  point  of  fusion  in  order  to  detect  the  effect,  but  it  is 
found  that  a  current  flows  between  the  positive  electrode  of 
a  platinum  wire  lamp  and  a  platinum  plate  placed  in  the 
vacuum  near  to  the  negative  end  of  that  wire. 

(8.)  The  material  of  which  the  plate  is  made  is  without 
influence  on  the  result.  Platinum,  aluminium,  and  carbon 
have  been  indifferently  employed. 

(9.)  The  active  agent  in  producing  this  effect  is  the  negative 
leg  of  the  carbon.  If  the  negative  leg  of  the  carbon  is  covered 
up  by  enclosing  it  in  a  glass  tube,  this  procedure  entirely,  or 
nearly  entirely,  prevents  the  production  of  a  current  in  a 
galvanometer  connected  between  the  middle  plate  and  the 
positive  terminal  of  the  lamp. 

(10.)  It  is  a  matter  of  indifference  whether  a  glass  or  metal 
tube  is  employed  to  cover  up  the  negative  leg  of  the  carbon ; 
in  any  case  this  shielding  destroys  the  effect. 

(11.)  If,  instead  of  shielding  the  negative  leg  of  the  carbon, 
a  mica  screen  is  interposed  between  the  negative  leg  and  ths 
side  of  the  middle  plate  which  faces  it,  then  the  current 
produced  in  a  galvanometer  connected  between  the  positive 
terminal  of  the  lamp  and  the  middle  plate  is  much  reduced. 
Under  the  same  circumstances  hardly  any  effect  is  produced 


122    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

when  the  mica  screen  is  interposed  on  that  side  of  the  metal 
plate  which  faces  the  positive  leg  of  the  carbon. 

(12.)  The  position  of  the  metal  plate  has  a  great  influence 
on  the  magnitude  of  the  current  traversing  a  galvanometer 
connected  between  the  metal  plate  and  the  positive  terminal 
of  the  lamp.  The  current  is  greatest  when  the  insulated 
metal  plate  is  as  near  as  possible  to  the  base  of  the  negative 
leg  of  the  carbon,  and  greatest  of  all  when  it  is  formed  into 
a  cylinder  which  embraces,  without  touching,  the  base  of  the 
negative  leg.  The  current  becomes  very  small  when  the 
insulated  metal  plate  is  removed  to  4  or  5  inches  from  the 
negative  leg,  and  becomes  practically  zero  when  the  metal 
plate  is  at  the  end  of  a  tube  forming  part  of  the  bulb,  which 
tube  has  a  bend  at  right  angles  in  it.  Copious  experiments 
have  been  made  with  metal  plates  in  all  kinds  of  positions. 

(13.)  The  galvanometer  current  is  greatly  influenced  by  the 
surface  of  the  metal  plate:  being  greatly  reduced  when  the 
surface  of  the  plate  is  made  small,  or  when  the  plate  is  set 
edgeways  to  the  negative  leg,  so  as  to  present  a  very  small 
apparent  surface  when  seen  from  the  negative  leg.  In  a  lamp 
having  the  usual  commercial  vacuum,  the  effect  is  extremely 
small  when  the  insulated  metal  plate  is  placed  at  a  distance 
of  18in.  from  the  negative  leg,  but  even  then  it  is  just  sensible 
to  a  very  sensitive  galvanometer. 

(14.)  If  a  charged  condenser  has  one  plate  connected  to  the 
insulated  metal  plate,  and  the  other  plate  connected  to  any 
point  of  the  circuit  of  the  incandescent  filament,  this  con- 
denser is  instantly  discharged  if  the  positively -charged  side  of 
the  condenser  is  connected  to  the  insulated  plate  and  the 
negative  side  to  the  hot  filament.  If,  however,  the  negative 
leg  of  the  carbon  horseshoe  is  shielded  by  a  glass  tube,  this 
discharging  power  is  much  reduced,  or  altogether  removed. 

(15.)  If  the  middle  plate  consists  of  a  separate  carbon  loop, 
which  can  itself  be  made  incandescent  by  a  separate  insulated 


ELECTRIC  GLOW  LAMPS.  123 

battery,  then,  when  this  middle  carbon  is  rendered  incandes- 
cent, and  is  employed  as  the  metal  plate  in  the  above  experi- 
ment, the  condenser  is  discharged  when  the  negatively-charged 
side  of  it  is  connected  to  the  hot  middle  carbon,  the  positively- 
charged  side  of  it  being  in  connection  with  the  principal 
carbon  horseshoe. 

(16.)  If  this  last  form  of  lamp  is  employed  as  in  (4),  the 
subsidiary  carbon  loop  being  used  as  a  middle  plate,  and  a 
galvanometer  being  connected  between  it  and  either  the 
positive  or  negative  main  terminal  of  the  lamp,  then,  when 
the  subsidiary  carbon  loop  is  cold,  we  get  a  current  through 
the  galvanometer  only  when  it  is  in  connection  with  the 
positive  main  terminal  of  the  lamp ;  but,  when  the  subsidiary 
carbon  is  made  incandescent  by  a  separate  insulated  battery, 
we  get  a  current  through  the  galvanometer  when  it  is  con- 
nected either  to  the  positive  or  to  the  negative  terminal  of 
the  lamp.  In  the  first  case  the  current  through  the  galvano- 
meter is  a  negative  current  flowing  from  the  middle  carbon 
to  the  positive  main  terminal,  and  in  the  second  case  it  is 
a  negative  current  from  the  negative  main  terminal  to  the 
middle  subsidiary  hot  carbon. 

(17.)  If  a  lamp  having  a  metal  middle  plate  held  between 
the  legs  of  the  carbon  loop  has  a  galvanometer  connected 
between  the  negative  main  terminal  of  the  lamp  and  this 
middle  plate,  we  find  that,  when  the  carbon  is  incandescent, 
there  is  no  sensible  current  flowing  through  the  galvano- 
meter. The  vacuous  space  between  the  middle  plate  and 
the  hot  negative  leg  of  the  carbon  possesses,  however,  a 
curious  unilateral  conductivity.  If  a  single  galvanic  cell  is 
inserted  in  series  with  the  galvanometer,  we  find  that  this 
ceH  can  send  a  current  deflecting  the  galvanometer  when 
its  negative  pole  is  in  connection  with  the  negative  main 
terminal  of  the  lamp,  but  if  its  positive  pole  is  in  connec- 
tion with  the  negative  terminal  of  the  lamp,  then  no  current 


124  ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

flows.  The  cell  is  thus  able  to  force  a  current  through  the 
vacuous  space  when  the  direction  of  connection  of  the  cell 
is  such  as  to  cause  negative  electricity  to  flow  across  the 
vacuous  space  from  the  hot  carbon  to  the  cooler  metal  plate, 
but  not  in  the  reverse  direction. 

(18.)  If  a  vacuum  tube  is  constructed,  having  horseshoe 
carbon  filaments  sealed  into  it  at  each  end,  and  which  can 
each  be  made  separately  incandescent  by  an  insulated  battery, 
we  find  that  such  a  vacuum  tube,  though  requiring  an  electro- 
motive force  of  many  thousands  of  volts  to  force  a  current 
through  it  when  the  carbon  loops  are  used  as  electrodes 
and  are  cold,  will  yet  pass  the  current  from  a  single  gal- 
vanic cell  when  the  carbon  loop  which  forms  the  negative 
electrode  is  rendered  incandescent.  It  is  thus  found  that 
a  high  vacuum  terminated  electrically  by  unequally-heated 
carbon  electrodes  possesses  a  unilateral  conductivity,  and 
that  electric  discharge  takes  place  freely  through  it  under 
an  electromotive  force  of  a  few  volts  when  the  negative 
electrode  is  made  highly  incandescent. 

Some  reference  must  be  made  before  concluding  this 
Lecture  to  the  evolution  of  the  high-voltage  incandescent  lamp, 
and  the  direction  in  which  invention  is  tending  with  respect 
to  the  utilisation  of  high-working  electromotive  forces  in 
electric  lighting. 

When  considering  the  subject  of  electric  distribution,  in 
the  last  Lecture,  it  will  be  pointed  out  that  there  are  strong 
arguments  in  favour  of  the  use  of  electric  pressures  as  high, 
or  higher,  than  200  volts  in  domestic  and  private  electric 
lighting  when  conducted  from  public  electric  supply  stations. 
Up  to  a  few  years  ago  the  usual  limit  of  electric  pressure 
for  working  incandescent  lamps  was  110  or  120  volts  at  the 
terminals  of  the  lamp.  The  clear  recognition  of  the  advan- 
tages to  be  gained  by  the  employment  of  200  or  2-50  volt 


ELECTRIC  GLOW  LAMPS.  125 

lamps  led  to  strong  representations  being  made  to  the 
Board  of  Trade  to  induce  them  to  raise  the  official  limit 
defining  the  minimum  pressure  which  might  he  introduced 
into  the  premises  of  a  user  of  electric  lamps  from  public 
electric  supply  stations.  The  then  existing  regulations  were 
accordingly  revised  by  the  Board  of  Trade,  and  it  is  now 
ordered  that  an  electric  pressure,  if  continuous  current,  of 
less  than  500  volts,  and,  if  alternating  current,  of  less  than 
250  volts,  shall  be  considered  as  a  low  electric  pressure. 
Systems  of  electric  supply  are,  therefore,  now  very  generally 
adopted  in  which  the  service  pressure  to  the  user  is  200  or 
250  volts.  A  demand  accordingly  sprung  up  for  an  incan- 
descent lamp  to  work  at  the  above  pressures. 

It  must  be  remembered  that  whilst  the  type  of  incandes- 
cent electric  lamp  most  largely  in  use  since  the  commence- 
ment of  electric  lighting  has  probably  been  the  16  c.p.  lamp, 
there  is  also  a  considerable  use  of  8  c.p.  lamps,  and  also  a 
considerable  demand  for  5  c.p.  lamps.  As  long  as  we  are 
limited  to  the  use  of  carbon  alone  in  the  lamp  filament  it  has 
not  hitherto  been  found  that  lamps  having  an  electrical  power 
consumption  at  a  less  rate  than  3  watts  per  caudle-power 
have  a  sufficiently  satisfactory  duration  on  ordinary  circuits  to 
render  them  widely  adopted  at  present.  The  reason  for  this 
is  that,  as  the  watts  per  candle  are  decreased,  the  temperature 
of  the  carbon  rises,  and  a  point  is  then  reached  at  which 
volatilisation  of  the  carbon  proceeds  very  rapidly.  Hence  we 
may  say  that  the  normal  type  of  16  c.p.  incandescent  lamp 
will  absorb  at  least  50  watts,  and  more  usually  60  watts,  whilst 
the  8  c.p.  lamps  and  5  c.p.  lamps  will  take  up  about  30  watts 
and  18  to  20  watts  respectively.  If,  then,  we  are  working  at 
an  electric  pressure  of  100  volts,  the  table  on  the  next  page 
shows  the  currents  taken  by  these  lamps  and  their  carbon 
filament  resistances  when  hot — that  is  to  say,  measured  when 
the  lamp  filament  is  incandescent.  Many  lamp  manufacturers 


126      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

issue  lamps  of  various  "  efficiencies  "  marked  for  voltages  so 
that  they  take  2  J,  3,  3  J  or  4  watts  per  candle.  Generally  speak- 
ing, when  used  on  ordinary  public  electric  supply  circuits,  the 
lower  the  "  watts  per  candle  "  the  shorter  the  life  of  the  lamp. 

Currents  and  Hot  Resistances  of  Carbon  Filament  Lamps  working 
at  100  volts  Pressure  and  3'5  watts  per  Candle-power. 


Candle-power 
of  lamp. 

Power  taken  up 
in  lamp  in  watts. 

Lamp  current 
in  amperes. 

Hot  resistance  of 
filament  in  ohms. 

16  c.p. 
8c.p. 
5c,p. 

560 
28-0 
17'5 

0-56 
0-28 
0175 

178 
356 
571 

It  will  be  seen,  therefore,  that  the  lower  the  candle-power 
of  the  100-volt  lamp  the  higher  must  be  the  resistance  of  the 
filament. 

This  necessitates  a  much  thinner  filament  in  the  case  of 
the  lower  candle-power  lamps,  assuming  all  to  be  made  for 
working  at  the  same  voHage.  The  proposal,  therefore,  to  use 
200-volt  lamps  ol  16,  8  or  5  c.p.  involved  the  construction 
of  lamps  taking  half  the  currents  of  the  above-mentioned  16, 8 
and  5  c.p.  100-volt  lamps,  and  having  filament  resistances 
four  times  as  great. 

In  the  manufacture  of  carbon  filaments  for  lamps,  it  is 
found  that  the  resistivity,  or  specific  resistance,  of  carbon 
is  reduced  by  all  processes  which  tend  to  make  a  very  dense 
or  tough  form  of  carbon.  Thus,  for  instance,  the  simple 
carbonisation  of  a  very  thin  strip  of  bamboo  (as  in  the  old 
form  of  Edison  lamp)  yields  a  carbon  which,  when  examined 
microscopically,  is  seen  to  be  of  a  porous  and  cellular 
structure;  in  fact  the  structure  of  the  vegetable  fibre  is 
retained  after  carbonisation  to  a  considerable  extent.  A 
carbon  of  this  kind  has  a  relatively  high  specific  resistance. 


ELECTRIC  GLOW  LAMPS.  127 

The  carbon  produced  by  carbonising  a  cotton  thread  which 
has  been  treated  with  dilute  sulphuric  acid  to  destroy  its 
organic  structure  and  produce  a  more  homogeneous  carbonis- 
able  structure  has  a  lower  specific  resistance  than  the 
simply  charred  organic  fibre.  All  modern  carbon  filament 
lamps  are  now  made  by  the  carbonisation  of  some  form  of 
soluble  cellulose  squirted  or  pressed  out  into  a  continuous 
thread  resembling  catgut.  Cellulose  is  dissolved  by  some 
suitable  solvent,  such  as  zinc  chloride  solution,  and  the 
resulting  gummy  mass  is  pressed  out  through  a  die  into  a  fine 
uniform  thread  or  wire.  This  material,  when  given  proper 
shape,  is  carbonised  in  a  furnace  out  of  contact  with  air. 
Carbons  made  in  the  above  manner  are  called  primary  carbons. 

It  is  usual,  however,  to  subject  the  primary  carbon  to  a 
process  called  "treating"  or  "flashing."  The  filament  is 
placed  in  a  vessel  enclosing  an  atmosphere  consisting  of  a 
hydrocarbon.  The  gas  generally  used  is  rarefied  coal  gas. 
If  the  carbon  filament  in  this  position  is  gradually  raised  to  a 
red  heat  and  finally  to  a  bright  white  heat  by  passing  an 
electric  current  through  it,  the  surrounding  hydrocarbon  gas 
or  vapour  is  decomposed  and  carbon  is  deposited  on  the 
filament.  Moreover,  if  the  primary  filament  is  non-uniform, 
and  has  defects  or  places  of  high  resistance,  the  current  will 
produce  heat  in  these  places  at  the  greatest  rate,  and  will, 
therefore,  raise  them  to  the  highest  temperature.  Accordingly, 
the  carbon-depositing  process  takes  effect  at  these  places 
most  quickly,  and  the  tendency  is,  to  some  extent,  to  remedy 
defects  in  the  primary  carbon  and  to  make  the  resulting 
filament  more  uniform  in  resistance.  Moreover,  by  con- 
trolling the  thickness  of  the  deposit  of  secondary  carbon, 
the  lamp  maker  can  adjust  the  resistances  of  filaments  so 
that  they  shall  be  of  exactly  the  required  resistance. 

This  "  deposited  carbon  "  has  a  much  denser  structure  than 
the  primary  carbon.  When  a  "  treated  "  filament  is  examined 


128      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

under  the  microscope  and  compared  with  an  "  untreated" 
filament  it  is  seen  that  the  surface  of  the  treated  filament  has 
a  bright  steely  lustre  and  has  a  very  uniform  and  metallic 
surface.  If  the  treating  has  been  badly  or  hastily  performed, 
or  carried  on  too  long,  the  surface  will  be  nodulous  or 
deformed  by  little  warts  or  projections  of  deposited  carbon. 
The  secondary  or  deposited  carbon  has  also,  per  se,  a  much 
lower  specific  resistance  than  any  form  of  primary  carbon 
produced  by  the  simple  carbonisation  of  an  organic  fibre.  A 
primary  carbon  can,  however,  be  prepared  with  a  dense  elastic 
structure  by  heating  an  organic  fibre  or  carbonised  fibre  to  an 
exceedingly  high  temperature  in  an  electric  furnace. 

Generally  speaking,  the  higher  the  temperature  at  which 
the  carbonisation  of  organic  material  has  been  conducted  the 
denser  the  form  of  carbon  produced,  and  the  lower  its 
specific  electrical  resistance.  At  very  high  temperatures  all 
forms  of  carbon  pass  into  graphite,  and  the  deposited  carbon 
or  electric  furnace-produced  carbon  consists  largely  of  carbon 
in  the  graphitic  form.  The  three  forms  of  carbon,  viz., 
charcoal,  graphite  and  diamond,  have  densities  which  are  very 
lifferent,  as  shown  by  the  following  table : — 

Density  or  Specific  Gravity  of  Carbon  in  Various  Forms. 
Water  =  1. 


Material. 

Density. 

Coke  

about  0'4. 

0-47  to  0-57. 

Pit  coal  

1-20  „  1-50. 

Carbonised  cellulose    .        

about  1-35. 

Anthracite  coal     ,  

1-41. 

4  '  Deposited  "  carbon 

1-95  to  2-28. 

Graphite    

2-10  „  2-35. 

Diamond    

3-3    ,,  3-55. 

It  will  be  seen  that  the  density  of  deposited  carbon,  or  the 
carbon  precipitated  on  a  filament  by  heating  it  in  coal  gas  or 


ELECTRIC  GLOW  LAMPS.  129 

benzol  vapour,  approximates  to  that  of  graphite.  The  density 
of  a  simply  carbonised  thread  or  bamboo  slip  is  similar  to 
that  of  most  varieties  of  coal. 

The  specific  electric  resistances  of  these  different  varieties 
of  carbon  vary  very  much.  The  diamond  is  an  exceedingly 
good  insulator.  Coke  and  charcoal,  in  the  more  porous  forms, 
are  very  poor  conductors,  but  graphite  has  an  electric  conduc- 
tivity which  places  it  almost  in  the  same  category  as  metals. 

In  lamp  manufacture  it  is  usual  to  state  the  electric 
resistance  of  filament  materials  per  inch-mil.  The  mil  is  the 
name  for  one-thousandth  of  an  inch.  If  a  very  fine  wire  is 
made,  of  circular  cross-section  and  one-thousandth  of  an  inch 
in  diameter,  this  is  said  to  be  one  mil  thick.  If  an  inch 
in  length  of  this  wire  is  cut  off,  the  piece  would  be  called 
one  inch-mil.  The  following  table  gives  the  electrical  resist- 
ances per  inch-mil  of  various  kinds  of  carbon  : — 

Electrical  Resistances  in  Ohms  per  inch-mil  of  Various  Forms 
of  Carbon. 

Small  arc  lamp  carbon  rod    , about  4,000  ohms 

Jablochkoff  candle  rod   „  2,000    „ 

Rod  of  Carre  carbon  1mm.  thick ,,  1,700    ,, 

Carbonised  bambo „  3,000    ,, 

Carbonised  parchmentised  thread    , ,  2, 000  to  2, 500  ohms 

Ordinary  carbon  filament,  "  treated  "  or 

flashed ,,  1,200 to  1,700    „ 

Deposited  or  secondary  carbon    ,,       300  to    450     ,, 

The  above  values  for  the  specific  resistance  of  various  carbons 
in  ohms  per  inch-mil  can  be  converted  into  microhms  per 
cubic  centimetre  sufficiently  nearly  by  multiplying  by  2. 

The  resistance  of  all  conductive  forms  of  carbon  diminishes 
as  temperature  increases.  The  very  interesting  observation 
has  been  recorded  by  Mr.  J,  W.  Howell  (The  Electrician, 


130      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

Vol.  XXXVIIL,  p.  836)  that  "treated"  carbon  filaments 
have  a  minimum  resistance  corresponding  to  a  certain 
temperature.  He  also  found  this  to  be  true  of  graphite. 

It  will,  therefore,  be  seen  that  the  graphitic  carbon 
deposited  upon  a  primary  carbon,  when  it  is  heated  in  coal 
gas  or  benzol,  has  a  specific  electrical  resistance  of  about  one- 
eighth  to  one- tenth  that  of  simple  carbonised  organic  material. 
Accordingly,  if  a  filament  is  treated,  its  total  or  average 
resistance  per  inch-mil  is  reduced  considerably  below  (gener- 
ally to  about  one-half)  the  value  which  it  had  before  treatment. 

In  constructing,  therefore,  a  very  high  resistance  filament, 
the  necessary  total  resistance  must  be  secured  by  making  the 
filaments  very  long.  It  is  not  possible  to  obtain  the  requisite 
durability  or  life  if  the  diameter  of  the  filament  is  reduced 
below  a  certain  value. 

The  practical  problem  in  the  construction  of  an  8  or  16  c.p. 
200-volt  lamp  is,  therefore,  to  place  in  a  bulb  of  the  usual 
size  a  sufficient  length  of  carbon  filament  to  give  the  desired 
length.  The  length  of  an  ordinary  16  c.p.  100-volt  carbon 
filament  is  about  5in.  and  it  is  about  8  or  10  mils  in  diameter. 
In  an  8  c.p.  lamp  the  length  will  be  somewhat  less  and  the 
diameter  of  the  carbon  will  be  about  7  mils.  The  conditions 
to  be  fulfilled  are  determined  by  the  surface  area  and  emis- 
sivity  of  the  carbon  at  the  temperature  to  which  it  is  raised 
when  incandescent,  and  by  the  resistivity  or  specific  resist- 
ance and  length  and  sectional  area  of  the  carbon  at  the  same 
temperature. 

When  a  treated  carbon  filament  of  circular  section  is  at  the 
highest  incandescence  it  can  endure  in  a  good  vacuum 
consistently  with  a  duration  of  an  average  life  of  1,000  hours 
or  so,  its  temperature  is  approximately  1,700°C.  At  this 


ELECTRIC  GLOW  LAMPS.  131 

temperature  its  intrinsic  brilliancy  is  such  that  it  is 
dissipating  about  half  a  watt  for  every  one-thousandth  of  a 
square  inch  of  total  radiating  surface.  This  rate  of  radiation 
brings  the  surface  to  a  state  of  incandescence  at  which  it 
emits  about  one  candle  light  for  every  25  ten-thousandths  of 
a  square  inch  of  apparent  surface.* 

Thus,  for  instance,  if  the  total  surface  of  a  round  carbon 
filament  is  1GO  square  inch  mils  (1  square  inch  mil  =  -001  of 
a  square  inch),  the  apparent  surface  which  radiates  in  any  one 
direction  is  about  50  square  inch  mils.  The  rate  of  dissipa- 
tion of  energy  is  then  80  watts  and  the  candle-power  about  20. 

Accordingly,  the  construction  of  a  high-voltage  carbon 
filament  lamp  of  normal  candle-power,  and  contained  in  a 
bulb  of  the  usual  size,  depends  very  much  upon  contrivances 
for  getting  the  requisite  length  of  carbon  filament  into  the 
bulb.  Since  primary  or  untreated  carbon  has  a  higher 
specific  resistance  than  treated  carbon,  some  makers  advocate 
the  use  of  an  untreated  carbon  for  use  in  high-voltage  lamps. 
It  is,  however,  recognised  that  the  softer  carbon  disintegrates 
or  volatilises  faster  than  the  treated  carbon ;  hence,  an  untreated 
filament  generally  deteriorates  faster  than  a  treated  filament. 

If  a  well-treated  filament  is  used  for  making  a  16  c.p.  230- 
volt  lamp,  about  14in.  of  carbon  filament  has  to  be  got  into 
the  bulb.  Lamp  makers  achieve  this  in  various  ways.  In 
some  cases  they  simply  place  two  double  loop  filaments  side  by 
side,  but  connected  in  series  in  one  bulb.  In  other  cases  they 
crinkle  or  corrugate  the  single- length  filament  (see  Fig.  54). 
In  order  to  prevent  the  long  carbon  loop  from  touching  the 
glass,  or  to  keep  the  various  turns  of  the  loops  from  contact 

*  The  apparent  surface  of  a  circular  sectioned  filament  is  equal  to  a 
rectangular  area  having:  the  same  length  as  the  filament,  and  a  width 
equal  to  its  diameter. 


132      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


ELECTRIC  GLOW  LAMPS.  133 

and  so  from  short-circuiting,  it  is  generally  found  essential  to 
hold  the  carbon  loops  in  position  by  stay-pins  of  platinum  wire 
let  into  the  glass  bulb.  In  any  case  the  carbon  filament  must 
be  sufficiently  stiff  when  hot,  or  appropriately  held,  to  prevent 
it  from  bending  over  and  touching  the  glass.  If  it  does 
touch  the  glass  will  be  cracked  and  the  vacuum  spoilt.  High- 
voltage  lamps  are  now  made  for  5,  8,  16  c.p.  and  upwards, 
up  to  working  voltages  of  250  c  p. 

In  the  low  candle-power  high-voltage  lamps  it  is,  however, 
necessary  to  make  the  filament  so  thin  that  it  has  not  a  very 
satisfactory  life  unless  used  with  a  perfectly  constant  service 
pressure. 

A  few  words  may  next  be  said  on  the  subject  of  the 
"  ageing  "  of  carbon  filament  lamps. 

It  is  a  familiar  fact  that,  after  a  certain  use,  carbon  lamps 
deteriorate  in  candle-power.  This  is  due  to  two  causes.  The 
carbon,  as  explained,  is  scattered  or  volatilised  and  deposited 
on  the  glass,  and  this  deposit  renders  it  more  opaque  to  light. 
Then,  also,  changes  take  place  in  the  filament — either  an 
increase  in  resistance  or  an  increase  in  surface — which  cause 
it  to  be  reduced  in  temperature  even  when  subjected  to  the 
same  terminal  voltage.  Generally  speaking,  it  is  found  that 
if  carbon  filament  lamps  are  worked  at  a  constant  voltage 
an  initial  rise  in  candle-power  takes  place  when  used,  and 
then  they  gradually  decay  in  candle-power.  The  initial  rise 
is  due  to  a  slight  resistance  of  the  filament,  which  makes 
the  lamp  to  take  more  current.  In  the  majority  of  cases, 
the  subsequent  decay  in  candle-power  is  due  entirely  to  the 
blackening  of  the  bulb.  If  the  filament  in  an  over-run  and 
blackened  lamp  is  remounted  in  a  new  bulb,  it  returns 
practically  to  the  same  candle-power  for  the  same  current 
as  at  the  beginning  of  its  life.  Hence  the  "  useful  life  "  of 


134      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

a  lamp  is  limited  by  the  rate  of  decay,  and  a  lamp  which 
has  deteriorated  20  per  cent,  in  candle-power  should  be 
reckoned  as  worked  out. 

This  rate  of  decay  depends  greatly  upon  the  ease  with 
which  the  carbon  surface  is  disintegrated,  hence  the  softer 
carbon  produced  by  the  simple  carbonisation  of  an  organic 
material  is  less  useful  than  "  deposited  carbon"  in  the  manu- 
facture of  filaments. 

Some  tests  have  shown  that,  on  the  average,  "  unflashed  " 
or  "untreated"  16  c.p.  filaments  lose  42  per  cent,  in  candle- 
power  and  35  per  cent,  efficiency,  or  120  candles  per  kilowatt, 
in  the  course  of  600  hours'  use. 


A  comparison  between  a  treated  cellulose  filament  and  an 
untreated  bamboo  filament  16  c,p.  lamp,  both  taking  3*20 
watts  per  candle  when  new,  showed  that  there  was  a  pro- 
gressive fall  in  candle-power  and  rise  in  watts  per  candle  as 
follows,  when  measured  at  regular  intervals  luring  the  life : — 

Table  showing  Typical  Decay  of  Treated  and  Untreated  Carbon 
Lamp  Filaments  with  Use. 


Hours' 
run. 

Candle-power. 

Watts  per  candle-power. 

Treated 
cellulose. 

Untreated 
bamboo. 

Treated 
cellulose. 

Untreated 
bamboo. 

0 

100 
200 
300 
400 
500 
600 
700 

16-0 
15-8 
15-86 
15-68 
15-41 
15-17 
14-96 
14-74 

16-0 
14-1 
12-9 
11-8 
11-0 
10-4 
9-9 
96 

3-16 
326 
313 
337 
3-53 
351 
354 
3-74 

3-20 
3-50 
3-80 
4-08 
4-32 
4-53 
4-75 
4-90 

To  meet  the  demand  for  a  high- efficiency  lamp — that  is,  a 
lamp  taking  less  than  8  watts  per  candle,  manufacturers  now 


ELECTRIC  GLOW  LAMPS, 


135 


generally  issue  lamps  of  various  efficiencies,  as,  for  example, 
lamp  filaments  marked  for  voltages  at  which  they  will  give 
light  at  the  rate  of  2-5,  3-0,  3-5  and  4-0  watts  per  candle. 
Lamps  working  at  less  than  3-0  watts  per  candle  are  usually 
called  high-efficiency  lamps.  Any  lamp,  however,  can  become 
a  high-efficiency  lamp  by  being  used  at  a  higher  pressure  than 
its  marked  volts.  Such  use,  however,  is  accompanied  by  a 
decreased  average  life. 

The  following  table  (from  the  Electrical  World,  Vol.  XXX., 
1897)  shows  the  results  of  some  tests  on  typical  3'1  watt 
lamps  in  producing  abbreviated  life  and  increased  efficiency 
when  the  lamps  were  run  at  voltages  above  the  normal. 

Tests  on  100-volt  Carbon  Filament  Lamps  when  run  at  Higher 
Voltages,  the  Lamps  having  normally  an  efficiency  of  3'1  watts 
per  Candle. 


Working 
voltage. 

Average 
life. 

Candle-power. 

Efficiency. 

100 
101 
102 
103 
104 
105 
106 

1,000 
818 
681 
662 
452 
374 
310 

100 
106 
112 
118 
125 
132 
140 

31 
30 

2'9 
28 
2-7 
26 
25 

For  the  sake  of  ease  of  comparison  the  normal  candle-power 
was  taken  at  100,  and  the  normal  life  as  1,000  hours,  when 
worked  at  the  marked  voltage  of  100,  and  the  other  values 
reduced  in  proportion. 

The  two  qualities  of  life,  or  duration  and  efficiency  or  watts 
per  candle,  are,  therefore,  closely  connected,  and  the  last 
cannot  be  changed  without  affecting  the  first.  It  is  obvious 
that  this  is  simply  due  to  the  greater  rate  of  volatilisation 
of  the  carbon  at  the  higher  temperature, 


136      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

To  obtain  high  efficiency,  that  is,  small  watts-per-candle, 
the  incandescent  material  must  be  used  at  a  very  high 
temperature,  and  it  is  obvious  that  the  limit  is  imposed,  in 
the  case  of  carbon,  by  the  fact  that  carbon  is  one  of  those 
substances,  like  iodine  and  camphor,  which  begin  to  pass 
into  a  state  of  vapour  at  temperatures  lower  than  their 
boiling  points.  Of  late  years  attention  has,  therefore,  been 
directed  to  the  use  of  other  materials  than  carbon  for  the 
manufacture  of  the  incandescing  filament  or  conductor. 

It  has  been  known  for  a  long  time  that  refractory  oxides, 
such  as  lime,  magnesia  and  other  analogous  substances, 
which  are  practically  non-conductors  when  cold,  become 
changed  into  fairly  good  conductors  when  heated  to  a 
high  temperature. 

A  small  cylinder  of  hard  lime  or  compressed  magnesia  is  a 
very  fair  non-conductor  when  cold,  but  if  heated  in  the 
oxyhydrogen  flame  or  in  an  electric  arc  it  becomes  changed, 
so  that  it  can  pass  current  under  the  operation  of  an 
electromotive  force  of  about  400  or  500  volts.  Dr.  Nernst  has 
lately  called  attention  again  to  this  fact,  and,  by  making 
small  rods  of  mixtures  of  refractory  oxides,  such  as  the 
oxides  of  thorium,  zinconium,  cerium,  yttrium  and  others, 
has  prepared  materials  which  can  become  sufficiently  con- 
ductive after  being  heated  in  a  flame  or  by  the  radiation 
from  an  incandescent  platinum  wire  to  pass  current  under 
electromotive  forces  of  100  or  200  volts. 

Lamps  of  this  description  require,  therefore,  a  preliminary 
heating  in  order  to  start  them  into  action.  Moreover,  since 
the  resistance  of  the  incandescing  material  decreases  rapidly 
as  the  temperature  rises,  the  oxide  rod  has  to  be  joined 
in  series  with  a  metallic  resistance  wire  in  order  that  the 
lamp  may  be  worked  in  parallel  with  others  on  constant 
pressure  circuits. 


ELECTRIC  GLOW  LAMPS. 

Owing  to  the  fact  that  refractory  oxides  of  the  earthy 
metals  can  endure  an  exceedingly  high  temperature  without 
volatilisation,  and  are  not  attacked  by  oxygen  at  a  high 
temperature,  it  is  not  necessary  to  enclose  the  oxide  rod  in 
an  exhausted  vessel,  and  also  it  is  possible  to  secure  for  a 
lamp  of  this  description  a  very  high  efficiency.  Experience 
is,  however,  as  yet  (July,  1899),  wanting  to  show  what 
would  be  the  durability  or  rate  of  radiative  decay  of  such 
oxide  lamps. 

Many  more  or  less  successful  attempts  have  been  made  to 
find  an  efficient  substitute  for  the  ordinary  carbon  filament  in 
an  incandescent  lamp  : — 

Dr.  C.  Auer  von  Welsbach  has  proposed  to  employ  a 
filament  of  osmium  or  of  platinum  on  which  is  placed  a 
deposit  of  thoria. 

Langhans  has  incorporated  silicon  with  carbon,  and  pre- 
pared a  carbide  of  silicon  filament. 

Maxim  has  prepared  a  very  dense  and  adamantine  form  of 
carbon,  and  innumerable  patents  have  been  taken  out  for 
incorporating  with  or  depositing  on  the  carbon  filament  some 
substances  more  refractory  than  carbon  and  less  volatile  at 
high  temperatures.  Up  to  the  present,  however,  no  really 
successful  substitute  has  been  found  for  a  well-manufactured 
carbon  filament  lamp  as  a  means  of  producing  light  by 
incandescence. 


LECTURE    III. 


THE  Forms  of  Electric  Discharge  :  Brush,  Glow,  Spark,  Arc. — Vacuum 
Tubes.— Sparking  Distance. — The  Electric  Arc. — The  Optical  Pro- 
jection of  the  Arc. — The  Arc  a  Flexible  Conductor. — The  High 
Temperature  of  the  Arc.— Non- Arcing  Metals. — Lightning  Protectors. 
— The  Distribution  of  Light  from  the  Arc. —  Continuous  and  Alter- 
nating Current  Arcs. — Voltage  Required  to  Produce  an  Arc. — The 
Physical  Actions  in  the  Arc. — The  Changes  in  the  Carbons. — The 
Distribution  of  a  Potential  in  the  Arc. — The  Unilateral  Conductivity 
of  the  Arc.— The  Temperature  of  the  Crater.— Comparison  with  Solar 
Temperature.— The  "Watts  per  Candle"  of  the  Sun.— Intrinsic 
Brightness  and  Dissipative  Power  of  Heated  Surfaces. — Comparison 
of  Glow  Lamp,  Arc  Lamp,  and  Sun,  in  respect  of  Brightness  and 
Radiation. — Arc  Lamp  Mechanism.— Arc  Lamp  Carbons. — The  Hissing 

of  Arc  Lamps. — The  Application  of 
Arc  Lamps. — Inverted  Arcs. — Series 
and  Parallel  Arc  Lighting.— The  En- 
closed Arc  Lamp. 

E  must  now  forsake  the  study  of 
the  incandescent  electric  lamp, 
and  turn  our  attention  to  the 
older  form  of  electric  illumina- 
tion, namely,  the  electric  arc 
lamp.  There  are  three  principal 
ways  in  which  electric  discharge 
seems  to  be  capable  of  taking  place  across  a  space  filled  with 
air  or  gas.  These  are  :  first,  by  the  glow  discharge ;  secondly, 
by  an  electric  spark,  or  disruptive  discharge ;  and  thirdly,  by 
the  electric  arc.  When  two  conductors  which  are  at  diffe- 
rent electric  pressures  are  brought  within  a  certain  distance 
of  one  another,  and  when  the  electric  pressure  difference 


140   ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

between  these  conductors  is  gradually  raised,  one  of  the 
above  three  modes  of  electric  discharge  is  always  established, 
and  takes  place  in  a  manner  depending  upon  the  gaseous 
pressure  of  the  air  or  gas  in  which  the  conductors  are 
immersed,  upon  the  electric  pressure  difference  between  the  con- 
ductors (called  the  electrodes),  and  upon  the  strength  of  the 
current  which  the  source  of  the  electric  pressure  can  pro- 
duce. If  the  two  conductors  are,  say,  fcwo  brass  balls 
which  are  connected  with  the  terminals  of  an  ordinary 
electrical  machine,  able  to  make  a  considerable  difference  of 
electric  pressure  between  them,  but  only  capable  of  gene- 
rating a  very  feeble  electric  current,  the  following  effects 
are  observed  when  the  balls  are  brought  near  one  ^another  : 
In  the  air,  at  ordinary  pressures,  the  conductors,  if  viewed 
in  the  dark,  will  be  found  to  be  covered  with  a  violet  glow 
of  light,  and  this  is  more  visible  if  their  ends  are  small — • 
if,  for  instance,  they  are  the  rounded  ends  of  two  wires 
attached  to  the  electrical  machine.  If  one  of  the  con- 
ductors has  a  large  surface — if,  for  instance,  it  is  a  brass 
plate — and  the  other  conductor  is  not  a  very  good  conductor 
—as,  for  example,  a  wooden  ball  or  knob — then  in  the  dark 
we  find  a  form  of  glow  discharge  taking  place  between  the 
ball  and  the  plate,  which  is  called  a  brush  discharge.  It 
is  a  violet,  broom- shaped  glow  of  light,  and  Sir  Charles 
Wheatstone,  on  examining  it  by  reflection  in  a  rapidly 
revolving  mirror,  showed  that  it  consisted  of  a  series  of 
very  rapid  electric  discharges  between  the  air  particles. 
It  is  generally  accompanied  by  a  slight  hissing  sound. 

This  form  of  brush  discharge  occurs  in  nature  under  some 
conditions,  and  is  known  by  the  name  of  St.  Elmo's  Fire. 
In  the  high  Alps,  when  in  the  neighbourhood  of  a  thunder- 
storm, travellers  have  often  observed  these  whizzing  brushes 
of  purple  light  proceeding  from  their  axes  and  alpenstocks, 
and  from  sharp-pointed  rocks;  and  a  similar  phenomenon 


ELECTRIC  AEG  LAMPS.  141 

lias  been  seen  proceeding  from  the  masts  and  yardarms 
of  ships  under  certain  electrical  conditions  of  the  atmo- 
sphere. When  the  knob  or  ball  is  positively  electrified  the 
electric  brush  is  larger  and  more  brilliant  than  when  the 
ball  is  negative.  The  finest  brushes  are  formed  in  nitrogen 
gas.  On  the  other  hand,  if  the  conductor  is  very  small, 
and  made  of  metal,  then  the  glow  that  is  seen  on  the  end 
of  this  conductor  in  the  dark  is  not  intermittent,  but  is 
associated  with  a  current  of  air  proceeding  from  the  con- 
ductor. A  blunt  metal  point  attached  to  an  electrical  machine 
can  thus  be  made  to  blow  out  a  candle  flame. 

The  form  and  nature  of  the  electric  discharge  in  rarefied 
gases  has  been  studied  by  many  observers.  The  phenomena 
are  exceedingly  complicated,  but  generally  speaking  are 
somewhat  as  follows  : — Into  the  extremities  of  a  closed  glass 
tube  or  bulb  are  sealed  platinum  wires,  which  can  be  con- 
nected to  an  electrical  machine  or  battery  or  any  source 
of  electric  current.  If  the  tube  is  nearly  exhausted,  or 
filled  with  highly  rarefied  gas  of  any  kind,  and  an  electric 
current  is  passed  through  it,  the  following  facts  can  be  noticed  : 
A  glow  of  light  surrounds  each  end  of  the  platinum  wire, 
and,  if  the  air  pressure  is  gradually  diminished  until 
the  air  is  rarefied  to  about  T Joth  part  of  its  ordinary 
pressure,  the  end  of  the  negative  electrode  or  conductor  is 
seen  to  be  surrounded  by  a  bluish  violet  light  separated 
from  the  electrode  by  a  dark  space  which  increases  in  width 
as  the  rarefaction  of  the  air  is  increased.  The  positive 
conductor,  or  electrode,  is  also  surrounded  with  a  glow  of 
light,  frequently  of  a  different  colour,  and  the  space  in 
between  may  be  filled  up  with  a  glow  of  light  which  may 
or  may  not  be  cut  up  by  a  series  of  dark  spaces,  and  is  then 
said  to  be  stratified.  This  phenomena  of  the  stratified  dis- 
charge can  be  shown  very  beautifully  by  connecting  the  two 
terminals  of  an  induction  coil  to  two  platinum  wires,  which 


142  ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


are  sealed  into  the  ends  of  a  long 
glass  tube  filled  with  rarefied  car- 
bonic acid  (Fig.  55).  The  tube  here 
used  is  one  of  a  large  collection 
which  belonged  to  the  late  Mr. 
Warren  de  la  Rue,  and  was  pre- 
sented to  this  Institution.  You  will 
see  that  the  space  in  between  the 
platinum  wires  is  filled  with  a 
light- coloured  violet  glow,  which  is 
stratified  or  cut  up  into  a  number 
of  saucer- shaped  layers  of  light. 
During  this  form  of  discharge  the 
negative  electrode,  or  wire  by  which 
the  current  leaves  the  tube,  is  found 
to  become  hot,  and  becomes,  in  time, 
raised  to  a  red  heat ;  whilst  at  the 
same  time,  it  is  more  or  less  disin- 
tegrated, depositing  a  film  of  metal 
upon  the  glass  in  its  neighbourhood. 

The  glow  discharge  in  gases  is 
a  very  complex  and  yet  attractive 
subject,  and  has  engaged  the  atten- 
tion of  numerous  able  physicists, 
such  as  Faraday,  Pliicker,  De  la 
Rive,  Hittorf,  Spottiswoode,  Moul- 
ton,  Crookes,  and,  more  lately, 
J.  J.  Thompson  and  many  others. 
It  is  found  that  this  phenomenon 
of  discharge,  both  in  rarefied  gases 
and  in  gases  at  ordinary  pressure, 
can  be  produced  not  only  by  the 
feeble  current  of  electricity  supplied 
by  an  ordinary  electrical  machine 


ELECTRIC  ARC  LAMPS.  143 

or  by  the  intermittent  current  of  an  induction  coil,  but  also  by 
the  continuous  current  given  by  a  voltaic  battery.  When  the 
electric  discharge  takes  place  in  air  at  ordinary  pressures,  and 
when  the  supply  of  current  is  not  very  rapid,  and  especially  if 
one  of  the  conductors  is  a  poor  conductor  of  considerable  sur- 
face, the  discharge  takes  the  form  of  a  series  of  electric  sparks. 
The  form  of  this  spark  and  the  distances  over  which  it  can  be 
produced  are  very  much  determined  by  the  nature  of  the  sur- 
faces. Bounded  metallic  knobs  give  bright  snapping  sparks, 
whereas  metallic  points  give  much  thinner  sparks.  Besides  the 
form  of  surface,  the  interposed  gases  affect  the  result.  Faraday 
found  that  different  gases  have  different  restraining  powers  upon 
the  spark.  At  the  same  distance  of  conductors,  hydrogen  gas 
permits  a  discharge  through  it  more  easily — that  is,  at  a  lower 
electric  pressure — than  air,  whereas  some  other  gases,  such  as 
hydrochloric  acid  gas,  have  peculiar  restraining  powers.  An 
electric  spark  is  in  reality  a  brief  current  of  electricity  taking 
place  between  the  two  conductors,  during  which  time  the  air 
or  other  gas  between  them  is  rendered  incandescent  and 
therefore  luminous,  and  portions  of  the  material  of  the  con- 
ductors between  which  the  discharge  is  taking  place  are 
carried  across  from  one  side  to  the  other.  For  each  particular 
pressure  there  is  a  certain  sparking  distance,  which,  however, 
is  partly  determined  by  the  form  of  the  conductors  between 
which  the  sparks  jump.  By  the  term  "  sparking  distance," 
corresponding  to  any  electric  pressure,  is  meant  the  distance 
over  which  the  spark  will  spring  when  the  difference  of 
electric  pressure  between  the  conductors  is  gradually  raised  to 
that  value.  If  we  bring  two  brass  balls,  connected  with  the 
terminals  of  an  electrical  machine,  within  a  certain  distance  of 
one  another,  and  then  darken  the  room,  you  will  see  these 
two  balls  surrounded  with  a  glow  of  light.  On  bringing  them 
within  a  certain  lesser  distance  of  one  another  a  snapping 
spark  passes,  which  is  repeated  at  intervals  as  the  machine  is 
worked. 


144    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


The  first  accurate  experiments  on  the  sparking  distance 
corresponding  to  different  electric  pressures  were  made  by 
Lord  Kelvin.  He  showed  that  between  slightly  curved  metal 
plates  it  required  a  pressure  of  about  5,000  volts  to  jump 
across  the  eighth  part  of  a  centimetre,  or  the  twentieth 
part  of  an  inch.  Mr.  De  la  Rue  found  that  between  flat 
plates  the  difference  of  pressure  required  to  produce  a  spark 
was  8,000  volts  when  the  surfaces  were  a  fifth  part,  of  a 
centimetre,  or  TJyths  of  an  inch,  apart;  and  11,300  volts 
when  the  surfaces  were  one-third  of  a  centimetre,  or  y^ths 
of  an  inch,  apart.  Similar  experiments  have  been  made  by 
numerous  other  physicists  with  like  results. 

The  following  table  of  results  for  sparking  distances  under 
certain  conditions  has  been  calculated  from  the  observations 
of  MM.  Bichat  and  Blondlot.  If  metal  balls  T4oths  of  an 
inch  in  diameter  (one  centimetre)  are  separated  by  a  distance 
beginning  at  ^\th  of  an  inch,  and  increasing  by  equal 
amounts — viz.,  by  one  millimetre  at  a  time — the  electric 
pressure  difference  in  volts  which  will  just  make  a  spark 
jump  across  at  the  various  distances  will  be  as  follows  (the 
distances  are  given  in  millimetres ;  one  millimetre  is  nearly 
^gth  of  an  inch) : — 


,    Distances  of 

Sparking 

Distances  of 

Sparking 

the  balls  in 
millimetres. 

pressure  in 

volts. 

the  balls  in 
millimetres. 

pressure  in 
volts. 

1 

4,765 

12 

27,024 

2 

8,140 

13 

27,765 

3 

11,307 

14 

28,359 

4 

14,119 

15 

28,949 

5 

16,664 

16 

29,363 

6 

19,210 

17 

29,837 

7 

21,823 

18 

30,133 

8 

22,792 

19 

30,547 

9 

24,153 

20 

30,932 

10 

25,071 

21 

31,198 

11 

26,255 

22 

31,494 

ELECTRIC  ARC  LAMPS.  145 

Hence  we  see  that,  between  such  metallic  balls,  an  electric 
pressure  of  30,000  or  40,000  volts  is  required  to  make  a  spark 
jump  over  a  distance  of  about  one  inch.  It  has,  however, 
been  shown  that  these  sparking  distances  are  greatly  affected 
by  the  state  of  polish  of  the  surfaces  and  by  their  form,  and 
that  sparking  is  promoted  by  light  of  particular  kinds  falling 
on  the  balls.  The  light  from  other  electric  sparks,  and  also  that 
from  burning  magnesium  wire,  has  a  strong  effect  in  breaking 
down  the  insulation  of  the  air  and  promoting  the  passage  of  a 
spark  between  polished  metal  balls  across  a  distance  which  it 
would  otherwise  not  pass.  Although  an  electric  spark  of  a 
few  inches  in  length  requires  for  its  production  an  electric 
pressure  of  thousands  of  volts,  it  conveys  but  a  small  quantity 
of  electricity.  It  is  somewhat  surprising  to  see  a  torrent  of 
noisy  sparks  passing  between  the  discharger  of  an  electrical 
machine,  and  yet  to  learn  that  this  represents  in  quantity, 
probably,  not  a  thousandth  of  a  coulomb,  and  that  it  could 
hardly  effect  the  chemical  decomposition  of  one  drop  of  water 
or  solution  of  a  metallic  salt,  although  its  pressure  or  potential 
may  be  hundreds  or  thousands  of  volts.  An  electric  pressure 
of  100  volts,  such  as  is  used  for  incandescent  lamps,  will 
hardly  produce  a  spark  over  any  visible  distance  ;  certainly  the 
distance  would  be  less  than  TJ^th  part  of  an  inch. 


These  various  forms  of  discharge  —  the  spark,  the  brush, 
and  the  glow  discharge  —  have  all  been  the  subject  of 
elaborate  investigations.  Faraday  particularly  studied  them 
in  the  twelfth  and  thirteenth  series  of  his  "  Experimental 
Researches  in  Electricity."  Whatever  be  the  means  by 
which  the  difference  of  electric  pressure  is  produced,  it  is 
found  that,  if  there  exists  a  sufficient  resistance  in  the 
circuit  limiting  the  rate  at  which  the  supply  of  electricity 
can  take  place,  then  no  other  form  than  the  spark,  brush, 
or  glow  discharge  is  possible.  If,  however,  we  employ 
a  primary  or  secondary  battery  —  that  is,  a  collection  of 


146      ELEUTBIC  LAMPS  AND  ELECTRIC  LIGHTING. 

voltaic  cells  —  for  producing  the  difference  of  pressure 
oetween  the  two  conductors,  such  an  arrangement  differs 
from  an  ordinary  electrical  machine  only  in  the  fact  that 
it  can  supply  a  stronger  or  more  powerful  electric  current. 
The  voltaic  battery,  when  compared  with  an  electrical 
machine  such  as  the  Wimshurst  machine,  must  be  thought 
of  as  a  very  large  pump  compared  with  a  very  small  one. 
Both  these  pumps  can  create  a  difference  of  level,  pumping 
up  water  from  one  level  to  a  cistern  or  reservoir  at  a  higher 
level,  but  the  pump  of  larger  capacity  differs  from  the  other 
in  that  it  can  supply  faster  and  can  keep  up  the  difference 
of  level  in  spite  of  out-flow  of  water.  Accordingly,  if  a 
large  number  of  voltaic  eels  are  joined  together,  and  the 
ends  of  this  battery  are  connected  to  two  brass  balls,  it  is 
found  that,  if  the  battery  is  one  which  has  a  high  internal 
resistance,  it  will  produce  a  series  of  small  sparks,  which 
jump  over  continually  from  one  ball  to  the  other,  provided 
the  difference  of  pressure  is  great  enough  and  the  balls  are 
brought  near  enough  together.  When  the  battery  is  one 
of  low  internal  resistance,  then  the  spark  discharge  cannot 
be  maintained ;  but  if  the  conductors  are  brought  near 
enough  together,  and  the  pressure  sufficiently  raised,  the 
spark  or  glow  discharge  passes  spontaneously  and  immedi- 
ately into  a  third  form  of  discharge,  which  is  called  the 
electric  arc. 

It  appears  that  this  arc  discharge  was  first  observed 
by  Curtet  in  1802,  only  two  years  after  the  invention  of 
the  battery  by  Volta ;  but  the  first  careful  study  of  the 
phenomena  of  the  electric  arc  in  air  and  in  vacuum  was 
made  by  Sir  Humphry  Davy  at  or  about  the  same 
time.  At  that  time  Sir  Humphry  Davy  was  engaged 
in  the  remarkable  series  of  electro-chemical  discoveries 
which  resulted  in  the  production  of  metallic  potassium  and 
sodium  from  caustic  potash  and  soda  for  the  first  time, 


ELECTRIC  ARC  LAMPS.  147 

Finding  the  necessity  for  a  larger  battery  than  he  possessed, 
he  laid  a  request  before  the  managers  of  the  Royal  Institu- 
tion in  1808,  asking  them  to  provide  the  means  of  con- 
structing a  battery  of  2,000  cells  with  which  to  continue  these 
researches.*  Shortly  afterwards  this  battery  was  provided, 
and  with  it  Davy  not  only  continued  his  electro- chemical 
discoveries,  but  studied  the  phenomena  of  the  electric  arc. 
Connecting  the  ends  of  this  large  battery  of  2,000  pairs 
of  plates  to  two  pieces  of  hard  charcoal,  which  had  been 
heated  and  then  plunged  into  quicksilver  to  make  them 
better  conductors,  Davy  observed  for  the  first  time  on  a 
large  scale  the  phenomena  of  the  electric  arc.  In  his 
"  Elements  of  Chemical  Philosophy "  (Vol.  IV.)  he  thus 
describes  the  first  production  of  this  very  large  2, 000- volt 
electric  arc : — 

"  The  most  powerful  combination  (battery)  that  exists  in 
which  number  of  alternations  (plates)  is  combined  with 
extent  of  surface  is  that  constructed  by  the  subscriptions 
of  a  few  zealous  cultivators  and  patrons  of  science  in  the 
laboratory  of  the  Royal  Institution.  It  consists  of  two 
hundred  instruments  connected  together  in  regular  order, 
each  composed  of  ten  double  plates  arranged  in  cells  of 
porcelain  and  containing  in  each  plate  32  sq.  in. ;  so  that 
the  whole  number  of  double  plates  is  2,000  and  the  whole 
surface  128,000  sq.  in.  This  battery,  when  the  cells  are 
filled  with  60  parts  of  water  mixed  with  one  part  of  nitric 

*  Sir  Humphry  Davy  laid  a  request  before  the  managers  of  the  Royal 
Institution  on  July  11,  1808,  that  they  would  set  on  foot  a  subscription 
for  the  purchase  of  a  large  galvanic  battery.  The  result  of  this  suggestion 
was  that  a  galvanic  battery  of  2,000  pairs  of  copper  and  zinc  plates  was 
set  up  in  the  Royal  Institution,  and  one  of  the  earliest  experiments  per- 
formed with  it  was  the  production  of  the  electric  arc  between  carbon  poles, 
on  a  large  scale.  It  is  probable,  however,  that  Davy  had  produced  the 
light  on  a  small  scale  some  six  years  before,  and,  according  to  Quetelet, 
Curtet  observed  the  arc  betweeu  carbon  points  in  1802.  See  Dr.  Paris' 
"  Life  of  Sir  H.  Davy." 

J.2 


148      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

acid  and  one  of  sulphuric  acid,  afforded  a  series  of  brilliant 
and  impressive  effects.  When  pieces  of  charcoal  about  an 
inch  long  and  one-sixth  of  an  inch  in  diameter  were  brought 
near  each  other  (within  the  thirtieth  or  fortieth  part  of  an 
inch)  a  bright  spark  was  produced  and  more  than  half  the 
volume  of  the  charcoal  became  ignited  to  whiteness,  and 
by  withdrawing  the  points  from  each  other  a  constant 
discharge  took  place  through  the  heated  air  in  a  space 
equal  at  least  to  4in.,  producing  a  most  brilliant  ascend- 
ing arch  of  light,  broad  and  conical  in  form  in  the  middle. 
Fragments  of  diamond  and  points  of  charcoal  and  plumbago 
rapidly  disappeared  and  seemed  to  evaporate  in  it,  but  there 
was  no  evidence  of  their  having  previously  undergone 
fusion." 

We  may  repeat  his  experiments  on  a  smaller  scale. 
Taking  two  round  rods  of  hard  compressed  carbon,  these 
are  connected  to  the  two  ends  of  a  battery  furnishing  a 
pressure  of  about  50  volts.  If  these  two  carbons  are  brought 
within  a  short  distance  of  one  another,  say  yj^th  of  an 
inch,  about  the  thickness  of  a  thin  visiting  card,  it  will  be 
found  that  this  electric  pressure  of  50  volts  is  not  sufficient 
to  create  a  spark  capable  of  jumping  over  this  distance.  The 
very  thin  layer  of  air  existing  between  the  two  carbon  ter- 
minals is  sufficient  to  insulate  entirely  this  pressure.  If  the 
two  carbons  are  touched  together  at  the  point  of  contact 
they  immediately  become  red  hot,  and  then,  on  separating 
them  again  a  slight  distance,  the  electric  discharge  continues 
to  take  place  between  them,  and  this  is  called  the  electric  arc. 
Instead  of  bringing  the  carbons  in  contact  we  can  break 
down  the  insulation  of  the  air  by  passing  a  small  electric 
spark  from  one  carbon  to  the  other,  or  even  at  some  distance 
away.  This  can  be  done  by  taking  a  discharge  from  a  small 
electrical  machine,  and  when  the  experiment  is  performed 
before  you  you  will  notice  that  the  electric  arc  discharge 


ELECTRIC  ARC  LAMPS.  149 

follows  immediately  upon  the  passage  of  a  spark  which  is 
too  small  to  be  visible. 

A  very  convenient  arrangement  for  producing  and  examin- 
ing an  electric  arc  is  one  in  which  one  of  the  carbon  rods  is 
fixed  and  the  other  is  moved  by  a  rack  and  pinion,  so  as  to  be 
brought  in  contact  with  the  first,  and  then  moved  away  as 
required  when  the  arc  is  produced  between  them.  This 
arrangement,  one  form  of  which,  as  devised  by  Mr.  Davenport,* 
is  shown  in  Fig.  56,  is  needed  in  an  experimental  study  of 
the  arc. 


Fia.  56.— Hand-Regulated  Arc  Lamp. 

In  order  to  view  more  comfortably  the  whole  phenomena 
that  are  taking  place  in  the  interior  of  this  dazzling  source 
of  light,  we  will  project  an  image  of  the  electric  arc  by 
means  of  a  lens  upon  the  screen,  and  examine  the  nature 
of  the  effects  taking  place  in  it.  The  arc  discharge  is  most 
easily  obtained  and  maintained  between  conductors  which 
are  good  but  not  very  good  conductors,  and  which  are  only 

*  This   convenient   form   of   hand- regulated  arc  lamp  was   described   in 
The  Electrician,  Vol.  XXXII.,  p.  393. 


150     XLSOTBIO  LAMPS  AMD  ELECTRIC 


volatilised  at  a  high  temperature.  It  can  easily  be  maintained 
between  certain  refractory  metals,  or  between  these  metals  and 
graphitic  carbon.  No  substance  has  been  found  superior  for 
the  purpose  of  producing  an  electric  arc  to  the  dense  variety  of 
carbon,  produced  either  from  the  graphitic  deposit  of  carbon 
found  in  the  interior  of  gas  coke  ovens,  or  by  the  production 
of  a  hard  variety  of  carbon  from  gas  coke,  lamp-black,  or 
sugar.  Twenty-five  years  ago  the  carbon  rods  which  were 
used  for  this  purpose  were  generally  produced  by  sawing 
lumps  of  very  hard  retort  carbon  into  small  rods  a  quarter 
of  an  inch  square,  but  of  late  years  an  immense  manufacture 
has  sprung  up  in  the  manufacture  of  arc-light  carbon  rods. 
In  the  manufacture  of  these  rods  some  form  of  dense  and 
very  pure  carbon  is  made  into  a  paste  with  syrup  or  coal  tar, 
and  from  this  rods  are  pressed  out  which,  after  being  dried 
and  baked  at  a  high  temperature,  furnish  carbon  rods  of 
various  sizes  which  are  used  for  the  production  of  the  electric 
arc.  These  carbon  rods  are  generally  sold  in  lengths,  varying 
in  size  from  a  quarter  of  an  inch  to  an  inch  in  diameter,  and 
from  nine  to  eighteen  inches  in  length.  Taking  two  of  these 
rods,  and  producing  between  them  an  electric  arc  in  a  closed 
lantern,  we  have  now  projected  upon  the  screen  an  enlarged 
image  of  the  electric  arc. 

Four  things  can  be  noticed  in  this  electric  arc  as  soon  as  it 
has  been  formed  by  bringing  the  carbons  into  contact  (see 
Fig.  57).  Looking  at  the  optical  image  of  the  arc  projected 
upon  the  screen,  we  notice  that  both  the  ends  of  the  two 
carbons  are  brilliantly  incandescent,  but  that,  if  the  arc  is 
formed  by  a  current  of  electricity  flowing  always  in  one 
direction,  called  a  continuous  current  arc,  then  the  positive 
carbon,  or  the  one  attached  to  the  positive  pole  of  the  battery 
or  dynamo,  and  marked  -f  in  the  illustration,  is  more 
brilliantly  incandescent  than  the  other.  This  positive  pole 
soon  has  its  end  hollowed  out  into  a  cup-shaped  depression, 


ELECT&IC  AHC  LAMPS.  151 

which  is  called  a  crater,  and  this  positive  or  crater  carbon, 
after  being  in  use  for  a  few  minutes,  will  be  found  to 
have  its  extremity  converted  into  graphitic  carbon  or  plum- 
bago, which  will  mark  upon  paper  like  a  pencil,  in  a 
way  that  it  would  not  do  before  being  so  used.  At  the 


FIG.  57. — The  Electric  Arc. 


same  time  the  negative  carbon  appears  to  get  more  pointed, 
and  becomes  surrounded  a  little  below  its  tip  with  small 
globules,  which  are,  in  all  probability,  condensed  carbon 
vapour.  If  the  arc  is  maintained  for  some  time,  it  will  be 


152    ELECTRIC  LAMPS  AND  fiLECTtilC 


found  that  both  these  carbons  are  being  worn  away  ;  but  the 
positive  or  crater  carbon  wears  away  about  twice  as  fast  as 
the  other  when  the  arc  is  formed  in  air.  In  the  space 
between  the  two  carbons  we  notice  a  band  of  brilliant  violet 
light,  which  is  surrounded  by  a  less  luminous  aureole  of  a 
golden  colour.  In  a  carefully-regulated,  steady  electric  arc  it 
will  be  seen  that  the  violet-coloured  portion  seems  to  spring 
from  the  white-hot  crater,  and  that  a  well-marked  dark  space 
separates  this  purple  part  from  the  golden-coloured,  wing- 
shaped  flames  which  appear  to  spring  from  the  negative 
carbon.  This  intermediate  central  portion  is  called  the  true 
arc,  and  hence  we  have  four  separate  portions  of  the  arc 
to  consider:  namely,  the  crater,  the  violet  -coloured  core  of 
the  arc,  the  aureole,  and  the  negative  carbon.  On  looking 
closely  at  an  arc  so  projected  upon  the  screen,  it  will  be 
seen  that  little  bits  of  carbon  are  continually  becoming 
detached  from  one  pole,  and  they  immediately  fly  over  to 
the  opposite  carbon.  It  can  be  shown  by  several  experiments 
that  there  is  probably  a  double  transport  of  material  going  on 
in  the  arc.  The  material  of  the  positive  pole  is  being  carried 
across  to  the  negative  side,  and  the  material  of  the  negative 
pole  is  being  carried  across  to  the  positive  gide. 

At  this  stage  it  will  be  interesting  to  analyse  the  light  of 
the  arc  by  means  of  the  prism.  Placing  a  vertical  slit  before 
the  lens  which  is  projecting  the  image  of  the  arc  upon  the 
screen,  we  cut  off  all  but  a  linear  band  of  light,  the  upper 
part  of  which  proceeds  from  the  crater  carbon,  the  middle  part 
from  the  violet  core  of  the  arc,  and  the  bottom  part  from  the 
negative  carbon.  The  prism  being  then  placed  in  front  of  the 
lens,  we  expand  this  linear  image  of  the  slit  into  a  spectrum 
which  is  divided  longitudinally  into  three  parts.  The  upper 
portion,  you  will  observe,  is  a  brilliant  continuous  spectrum  of 
light,  in  which  all  the  prismatic  rays  are  present.  This  spec- 
trum proceeds  from  the  light  emitted  from  the  crater  of  the  arc. 


ELECTRIC  ARC  LAMPS.  153 

The  middle  portion  of  the  spectrum  is  much  less  bright 
in  the  orange,  red,  yellow  and  green,  but,  as  you  will 
see,  is  distinguished  by  two  remarkably  brilliant  violet  bands, 
which  are  characteristic  bands  in  the  spectrum  of  carbon 
vapour.  The  bottom  of  the  spectrum,  formed  from  the 
negative  carbon,  is  a  continuous  spectrum  similar  to  the 
upper  one,  only  less  brilliant.  On  lengthening  out  the  arc 
by  drawing  the  carbons  apart,  we  find  that  the  two  upper 
spectra  move  away  frcji  one  another,  the  middle  spectrum 
increasing  in  width.  At  a  certain  point  the  arc  breaks  down, 
the  middle  spectrum  with  its  two  violet  bands  disappearing 
instantly;  but  for  a  short  period  of  time  the  upper  and 
under  spectra  linger  as  the  carbons  cool,  because  the  carbons 
continue  to  glow  faintly  ;  and  as  these  carbons  cool  you 
observe  the  two  spectra  shrinking  rapidly  from  the  violet 
end,  until  at  last  only  faint  traces  of  the  red  and  green 
remain,  and  these  finally  vanish.  But  you  will  see  that 
the  lower  spectrum,  which  proceeds  from  the  light  of  the 
negative  carbon,  vanishes  first.  This  only  indicates  that 
the  crater  carbon  has  a  much  higher  temperature  than  the 
negative  carbon,  and  is  in  accordance  with  all  that  we 
know  about  the  arc. 

If  we  examine  an  electric  arc  of  this  kind,  we  notice  that, 
as  the  carbons  are  gradually  drawn  apart  and  the  arc  length- 
ened, the  aureole  of  golden  vapour  increases  and  forms  a  sort 
of  large  lambent  flame,  which  disappears  when  the  arc  is 
lengthened  beyond  a  certain  point.  After  such  a  continuous 
current  arc  has  been  extinguished,  we  can  always  tell  which 
has  been  the  positive  carbon,  not  only  from  the  crater  hol- 
lowed in  it,  but  also  from  the  fact  that  it  remains  red  hot  for 
a  longer  time  than  the  negative  carbon.  The  exceedingly  high 
temperature  of  the  positive  carbon  is  shown  by  the  fact,  above 
mentioned,  that  after  being  used  for  some  time  it  is  converted 
into  graphite  at  the  extremity  and  will  mark  paper. 


154    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

The  electrical  power  taken  up  in  the  continuous  current 
electric  arc  is  measured  in  an  exactly  similar  way  to  the  power 
taken  up  and  dissipated  in  the  filament  of  an  incandescent  lamp 
— namely,  by  measuring  the  current  in  amperes  flowing  through 
the  arc,  and  the  difference  of  electric  pressure  in  volts  between 
the  carbons.  Multiplying  these  numbers  together,  we  obtain 
the  value  of  the  power,  measured  in  watts,  being  absorbed  in 
the  arc.  By  far  the  best  way  to  denominate  electric  arcs 
is  by  the  power  in  watts  absorbed.  We  may  thus  speak 
of  a  300- watt  arc,  or  a  500- watt  arc,  or  of  a  1,000- watt  arc. 
The  length  of  the  arc  can  be  measured  most  conveniently 
by  projecting  the  image  of  the  carbons  upon  the  screen. 
If  a  sheet  of  polished  metal  is  then  held  behind  the  arc,  a 
faint  diffused  light  will  be  thrown  upon  the  screen,  which 
will  enable  us  not  only  to  see  the  arc,  but  to  see  also  the 
image  of  the  two  carbon  rods.  By  means  of  a  pair  of  com- 
passes or  a  foot  rule  we  can  measure  the  length  of  the 
magnified  image  of  the  arc — that  is  to  say,  the  distance 
between  the  image  of  the  tips  of  the  carbon  rods.  We  can 
also  measure  the  width  of  the  magnified  image  of  either  of 
the  carbon  rods.  Let  us  suppose  that  the  rods  are  each  of 
them  half  an  inch  in  diameter,  and  that  their  image  upon 
the  screen  is  eight  inches  in  diameter ;  then  the  arc  has 
been  magnified  16  times.  If,  then,  we  find  that  the  image 
of  the  arc  is,  say,  two  inches  in  length  when  measured  upon 
the  screen,  it  shows  us  that  the  real  length  of  the  arc  is 
Y^th  part  of  this,  namely,  one- eighth  of  an  inch,  because 
the  length  of  the  arc  is  magnified  in  the  same  ratio  as 
the  width  of  the  carbons.  This  optical  method  of  measur- 
ing the  length  of  the  arc  affords  a  very  easy  means  of 
accurately  ascertaining  the  length  of  the  arc  without  danger 
or  difficulty.  Electric  arcs  may,  roughly,  be  distinguished  as 
long  arcs  and  short  arcs,  according  to  the  distance  between 
the  carbon  poles,  although  the  dividing  line  between  the 
two  is  not  in  any  way  accurately  determined. 


ELECTRIC  AUC  LAMPS.  155 

Before  passing  on  to  consider  the  physical  measurements 
connected  with  the  electric  arc,  let  us  notice  that  the  violet 
core  of  the  arc,  which  consists  of  a  torrent  of  incandescent 
carbon  vapour,  acts  like  a  perfectly  flexible  conductor.  If  I 
bring  a  magnet  near  to  such  an  electric  arc,  you  will  notice 
that  the  magnet  apparently  impels  the  arc  sideways,  and  if  it 
is  brought  near  enough  it  will  actually  blow  it  out.  The 
reason  for  this  is  because  the  arc,  as  a  conductor  traversed  by  a 
current,  exert0  an  electro-magnetic  force  upon  the  magnet, 
and  in  like  manner  the  magnet  exerts  an  electro-magnetic 
force  upon  the  arc.  The  arc  always  tends  to  move  across  the 
lines  of  force  of  the  magnet,  and  in  this  way  may  actually 
be  bent  into  a  bow  shape,  or  be  drawn  out  into  a  kind  of  blow- 
pipe flame. 

Brief  references  must  now  be  made  to  the  generation  of 
light  and  heat  in  the  arc.  Experiment  seems  to  show  that  in 
the  continuous  current  arc — that  is,  the  arc  produced  by  a 
current  of  electricity  always  flowing  continuously  in  the 
same  direction — by  far  the  larger  proportion,  perhaps  85 
per  cent.,  of  the  light  thrown  out  comes  from  the  highly 
incandescent  crater  carbon,  about  15  per  cent,  comes  from 
the  negative  carbon,  and  very  little  indeed,  certainly  not 
more  than  5  per  cent.,  from  the  true  electric  arc.  The  great 
source  of  light,  therefore,  in  the  electric  arc  is  the  intensely 
luminous  crater  formed  at  the  end  of  the  positive  carbon,  which 
is  brought  up  to  an  exceedingly  high  temperature.  Various 
estimates  have  been  made  of  the  temperature  of  this  electric 
crater,  but  the  physical  difficulties  connected  with  these 
experiments  are  very  great.  Bossetti  in  1879  stated,  as  the 
result  of  some  of  his  experiments,  that  the  positive  carbon 
had  a  temperature  of  3,200°C.,  and  the  negative  carbon 
2,5000C.  More  recently,  Violle  has  given  a  temperature 
of  3,600°C.  as  the  temperature  of  the  crater  carbon,  and 
other  observers  have  given  even  higher  results  than  these. 


156    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

This  temperature  is  sufficient  not  only  to  melfc  but  actually 
to  boil  away  even  the  most  refractory  metals.  We  can  easily 
show  the  high  temperature  of  the  crater  by  boiling  in  it 
a  piece  of  copper  or  silver.  If  an  electric  arc  is  formed  with 
the  crater  downwards,  and  a  small  piece  of  copper  is  placed  in 
the  crater,  and  then  the  image  of  the  arc  is  projected  upon 
the  screen,  you  will  see  the  little  piece  of  copper  boiling 
violently  and  sending  out  a  torrent  of  copper  vapour,  which, 
in  its  incandescent  condition,  emits  a  brilliant  green  light.  In 
the  same  way,  silver,  platinum,  and  even  the  refractory  metal 
iridium,  can  be  melted  and  boiled  into  vapour  in  the  electric  arc. 
It  has  been  already  mentioned  that  there  is  a  transport  of 
material  in  the  arc,  material  passing  over  from  the  positive  to 
the  negative  pole,  and  vice  versa.  It  is  only  natural,  therefore, 
to  expect  that,  since  the  electric  arc  is  a  phenomenon  of  electric 
discharge  taking  place  in  a  highly -conducting  vapour,  the 
longest  arcs  will  be  capable  of  being  maintained  between  the 
'  most  volatile  metals,  and  experience  shows  this  to  be  the  case 
generally.  It  has  been  found  that  the  difference  of  tempera- 
ture between  the  two  poles  is  greater  in  proportion  as  they 
are  worse  conductors  and  more  easy  of  disaggregation  ;  also 
that,  to  produce  an  arc  of  a  given  length  requires  a  greater 
current  in  proportion  as  the  infusibility  of  the  electrode  is 
greater. 

A  very  striking  fact  was,  however,  discovered  by  Wurts, 
viz.,  that  there  are  four  metals — zinc,  cadmium,  mag- 
nesium and  mercury — which  are  non-arcing  metals,  so 
called  because  it  is  difficult  or  impossible  to  maintain  an 
alternating-current  electric  arc  between  the  surfaces  of  these 
metals  which  are  very  near  together.  This  fact  is  taken 
advantage  of  in  the  production  of  a  very  ingenious  lightning 
protector,  which  may  just  briefly  be  described.  When  over- 
head circuits,  consisting  of  copper  wires  carried  on  insulators 
on  posts,  are  employed  for  the  distribution  of  alternating 


ELECTRIC  ARC  LAMPS.  157 

electric  currents,  such  overhead  lines  are  very  much  exposed 
to  lightning  stroke.  If  they  should  be  struck  by  lightning, 
the  lightning  discharge  will  generally  pass  back  into  the 
generating  station,  and  do  damage  to  the  dynamo  machines. 
In  order  to  prevent  this,  a  lightning  protector  of  the 
following  kind  has  been  designed,  especially  for  use  with  over- 
head alternating- current  circuits,  the  object  of  which  is  to 
allow  the  lightning  striking  the  overhead  circuits  to  pass  to 
earth,  but  yet  at  the  same  time  to  prevent  the  current  from 
the  dynamo  machine  following  it.  On  a  slab  of  marble  or 
porcelain  are  placed  an  odd  number  of  small  zinc  cylinders, 
one  inch  in  diameter  and  about  three  inches  long.  These 
cylinders  are  placed  close  to  one  another,  being  separated 
by  a  distance  only  of  -^th  of  an  inch.  Each  cylinder  is 
insulated.  The  middle  cylinder  is  connected  by  a  wire  with 
a  plate  sunk  in  the  earth.  The  two  outside  cylinders  are 
connected  to  the  two  lines  which  form  the  overhead  circuit. 
If  the  lightning  strikes  the  line  on  either  side,  the  discharge 
jumps  over  from  cylinder  to  cylinder  until  it  reaches  the 
middle  cylinder,  and  then  it  goes  to  earth  by  the  earth-plate. 
In  so  doing  it  will  attempt  to  start  an  electric  arc  between  the 
cylinders ;  but,  as  a  permanent  arc  cannot  be  maintained 
between  zinc  surfaces,  this  arc  is  almost  instantaneously  ex- 
tinguished, and  the  dynamo  machine,  therefore,  is  unable  to 
produce  an  electric  arc  across  between  the  two  mains.  In 
this  way  a  safe  passage  for  the  lightning  to  earth  is  provided. 

It  will,  in  the  next  place,  be  necessary  to  direct  attention 
to  the  distribution  of  light  from  an  electric  arc.  On 
examining  an  electric  arc  formed  by  a  continuous  current, 
the  positive  or  crater  carbon  being  uppermost,  a  very  cursory 
examination  shows  us  that  the  light  sent  out  from  such 
an  arc  is  not  uniform  in  all  directions.  Very  little  light  is 
sent  upwards.  By  far  the  larger  portion  of  the  light  is  sent 
downwards  at  an  angle  of  from  30  deg.  to  40  deg.  below 


158    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


the  horizon.  By  the  aid  of  the  explanations  given  above,  it  is 
very  easy  to  see  the  reason  for  this.  It  is  really  due  to  the 
fact  that  the  greater  portion  of  the  light  is  sent  out  from  the 
bottom  surface  of  the  top  carbon ;  hence,  when  viewed  in  a 
horizontal  position,  but  very  little  of  this  incandescent  crater 
can  be  seen.  On  raising  the  arc,  so  as  to  look  up  underneath 
it,  the  amount  of  apparent  surface  of  the  crater  which  is  seen 
increases  up  to  a  certain  point  as  we  elevate  the  arc.  But 
if  the  arc  is  raised  beyond  a  certain  amount  above  the  eye, 
then  the  bottom  or  negative  carbon  begins  to  stand  in  the  way  of 
the  light  sent  out  from  the  crater,  and  to  diminish  the  apparent 


Fro.  58.— Model  showing  the  Relative  Areas  of  Crater  seen  at  different 
angles  of  view. 

area.  By  taking  two  cylinders  of  pasteboard,  N  and  P,  held 
by  threads  a  little  distance  from  one  another  (see  Fig.  58)  and 
fixing  on  the  underneath  end  of  the  upper  cylinder  a  disc  of 
red  paper  to  represent  the  incandescent  crater,  we  may  easily 
convince  ourselves,  by  looking  at  this  model  arc  in  different 
directions,  that  there  is  a  certain  position  in  which  the  eye  can 
be  placed  in  which  it  will  see  the  largest  apparent  area  of  the 
crater. 

There  is,   therefore,   a  position  in   which  the  maximum 
amount  of  light  will  be  thrown  out  by  a  real  arc  represented 


ELECTRIC  AEG  LAMPS. 


159 


by  this  model.  Accordingly,  if  the  intensity  of  the  light 
given  by  an  electric  arc  is  measured  in  different  directions, 
say  for  every  ten  degrees  of  inclination  above  and  below 
the  horizontal,  we  find  different  photometric  values  for 
these  rays  in  these  directions.  We  can  represent  this 
different  illuminating  powers  in  different  directions  by  a 
curve  (see  Fig.  59),  which  is  called  the  photometric  curve  of 
the  arc.  The  magnitude  of  the  radii  of  this  curve,  taken  in 
different  directions,  represent  the  intensity  of  the  light  pro- 


•  720° 


Fia.  59. — Curve  showing  the  Distribution  of  Light  from  a  Continuous 
Current  Arc.  The  figures  printed  on  the  radial  lines  represent  the  candle- 
power  in  different  directions. 

ceeding  from  the  arc  in  these  directions.  The  exact  position 
of  the  maximum  intensity  will  depend  upon  the  length  of  the 
arc,  the  size  of  the  crater,  and  the  relative  widths  of  the  nega- 
tive and  positive  carbons.  Accordingly,  in  street  arc  lighting 
advantage  is  taken  of  this  fact.  If  the  electric  arcs  are 
put  up  on  high  poles,  or  street  supports,  the  positive  or 
crater  carbon  being  uppermost,  they  will  throw  down  their 


ICO     ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

maximum  amount  of  light  in  a  direction  inclined  at  40  or 
50  degrees  to  the  horizon,  and  illuminate  a  circular  zone 
around  them  with  a  maximum  intensity.  Just  underneath 
the  arc  there  will  be  more  or  less  shadow,  and  in  a  direction 
above  the  horizon  very  little  light  will  be  thrown  out. 

In  order  to  obtain  the  best  results  and  the  most  uniform 
distribution  of  light,  engineers  have  found  it  best  to  use  a 
very  small  negative  carbon,  in  order  that  the  negative  carbon 
may  stand  as  little  as  possible  in  the  way  of  the  light  sent  out 
from  the  crater  in  a  downward  direction,  and  that  the  arc  may, 
therefore,  illuminate  the  largest  possible  area  underneath  it. 
This  unequal  distribution  of  the  light  from  the  arc  can,  to  a 
great  extent,  be  modified  by  the  use  of  a  somewhat  peculiar 
glass  globe ;  but  we  shall  point  out  in  an  instant  a  method  by 
which  superior  results  are  gained  for  internal  lighting. 

JN  ot  only  can  we  form  an  electric  arc  by  means  of  a  continuous 
current  of  electricity — that  is  to  say,  one  always  flowing  in  the 
same  direction — but  we  can  employ  an  alternating  current  of 
electricity.  In  such  an  alternating  current  there  is  a  very 
rapid  change  in  the  direction  of  the  electric  current,  the 
current  flowing  first  in  one  direction  for  a  short  period,  then 
being  arrested,  and  then  flowing  in  the  other  direction  for  an 
equally  short  period.  An  alternating  current  of  electricity 
may  therefore  be  compared  to  the  ebb  and  flow  of  water  in  a 
tidal  river,  in  which  the  water  flows  in  one  direction  for  a 
certain  number  of  hours  and  is  then  reversed  and  flows  in 
the  opposite  direction  for  about  an  equal  period.  A  continuous 
current  of  electricity  may  be  likened,  on  the  other  hand,  to  a 
non-tidal  river  in  which  the  water  flows  always  in  one  direction. 

In  what  are  called  alternating  current  systems  of  electric 
lighting  distribution  it  is  very  usual  to  employ  an  alter- 
nating current  having  a  frequency  of  from  40  to  100  periods 


ELECTKIC  AEG  LAMPS. 


161 


per  second — that  is  to  say,  the  electric  current  changes 
its  direction  from  80  to  200  times  in  one  second.  We 
can  form  an  electric  arc  not  only  with  a  continuous  but  with 
an  alternating  current  of  electricity,  and  the  reason  for  this  is 
that  there  is  a  certain  persistence  in  the  electric  arc.  It  is 
found  that,  when  the  arc  is  made  with  a  continuous  current  of 
electricity,  the  current  can  be  interrupted  for  a  short  fraction 
of  a  second  without  putting  out  the  arc.  Hence  the  current 
of  electricity  can  be  reversed  in  direction  without  extinguish- 


'  80° 


Yo° 


-'60° 


FIG.  60. — Curve  Showing  the  Distribution  of  Light  from  an  Alternating 
Current  Arc.  The  figures  marked  on  the  the  radial  lines  indicate  candle- 
power  in  these  directions. 

ing  the  arc.  If,  however,  it  is  reversed  very  rapidly,  then  the 
result  will  be  that,  instead  of  having  one  positive  and  one  nega- 
tive pole  or  carbon,  each  carbon  will  be  alternately  positive  and 
negative,  and,  instead  of  having  only  one  crater,  we  shall  have 
both  carbons  cratered  out  and  sending  out  light.  An  alternating 
current  arc  produces,  therefore,  a  very  different  distribution  of 
light  from  a  continuous  current  arc.  In  an  alternating  current 


162     ELEGTRXQ  LAMPS  AND  ELECTRIC  LIGHTING. 

arc  the  light  is  not  only  sent  downwards,  but  it  is  sent  upwards 
as  well,  the  distribution  of  light  being  found  to  be  about 
equal  in  directions  above  and  below  the  horizon.  The  curve 
representing  the  distribution  of  the  light  of  an  alternating  arc 
lamp  is  shown  in  Fig.  60,  from  which  it  will  be  seen  that  the 
alternating  current  arc  sends  its  maximum  illumination  in 
directions  both  upwards  and  downwards  equally  inclined  to 
the  horizon.  Objections  are  sometimes  taken  to  the  use 
of  an  alternating  current  arc  on  the  ground  that  it  makes 
a  disagreeable  noise  or  humming.  This,  however,  is  much 
less  evident  in  some  arcs  than  in  others,  and,  by  the  employ- 
ment of  suitable  carbons  and  a  proper  frequency  of  current, 
can  very  nearly  be  abolished.  It  appears  to  be  proved, 
however,  that  an  alternating  current  arc  does  not  yield  the 
same  total  illumination  for  a  given  expenditure  of  energy 
in  it  as  does  a  continuous  current  arc. 

Eeturning  then  to  the  consideration  of  the  continuous - 
current  arc,  we  may  note  further  physical  facts  connected 
with  its  production  between  various  terminals.  If  a  con- 
tinuous-current arc  is  formed  between  carbon  rods,  and  if 
the  image  of  the  arc  is  projected  on  a  screen  so  that  its 
length  can  be  measured,  and  if,  at  the  same  time,  the 
current  through  the  arc  and  the  potential  difference  (P.D.) 
of  the  carbons  are  ascertained,  it  is  found  that  there  is  a 
definite  relation  between  the  length,  current,  and  potential 
difference  which  can  only  be  properly  expressed  by  a  series 
of  curves. 

One  important  fact  in  connection  with  the  continuous- 
current  electric  arc  is  that,  if  the  current  is  increased  beyond 
a  certain  value,  the  arc  passes  into  an  unstable  condition 
in  which  it  begins  to  hiss.  In  the  case  of  the  open  arc 
this  is  accompanied  by  a  sudden  fall  in  arc  voltage,  or 
P.O.,  of  about  10  volts.  On  setting  out  as  curves  the 


ELECTRIC  ARC  LAMPS.  163 

relation  of  arc  length,  current  and  voltage,  it  is  found  that 
there  is  a  certain  minimum  voltage  (approximately  30  or  35 
volts)  below  which  no  true  arc  of  any  finite  length  can  be 
formed.  This  initial  voltage  has  sometimes  been  called  the 
counter-electromotive  force  in  the  arc.  There  are  many 
reasons,  however,  which  make  it  impossible  to  believe  that 
in  the  electric  arc  there  is  a  true  source  of  electromotive 
force  working  against  the  external  impressed  electromotive 
force. 

This  initial  or  non-effective  voltage  must  rather  be  regarded 
as  one  of  the  factors  in  the  expression  for  the  energy  which 
has  to  be  imparted  to  the  arc  to  keep  up  a  supply  of  carbon 
vapour.  The  processes  which  go  on  during  the  formation 
and  maintenance  of  a  continuous- current  electric  arc  appear 
to  be  something  as  follows :  A  large  number  of  facts  seem 
to  indicate  that  electric  conduction  through  vapours  or  gases 
is  in  reality  electrolytic  in  character — that  is  to  say,  it 
depends  upon  a  continual  breaking  up  of  molecules  of  the 
gas  into  something  smaller,  possibly  atoms,  which  are  called 
ions.  These  ions  carry  electric  charges,  and  when  a  mole- 
cule is  broken  up  into  two  ions  these  ions  have  opposite 
charges,  and  move,  probably,  with  different  velocities,  in 
opposite  directions  in  the  electric  field.  A  similar  process 
has  long  been  recognised  as  taking  place  in  the  case  of 
conduction  through  certain  compound  liquids.  If  this  is 
the  case,  the  column  of  carbon  vapour,  which  forms  the  true 
electric  arc,  is  a  region  in  which  resolution  of  carbon  mole- 
cules into  carbon  ions  is  takiog  place,  and  these  positive  and 
negative  ions  are  being  caused  by  the  electric  force  in  the 
space  to  move,  probably  with  unequal  speed,  in  opposite 
directions. 

If  an  arc,  formed  with  solid  carbon  rods,  is  allowed  to 
bum  quietly  for  some  time,  and  if  the  current  is  not 
increased  to  the  hissing  point,  we  have  the  condition  called 

M2 


164     tiLEOTiliG  LAMPS  AND  tiLECTRIC  LIGHTING. 

the  normal  arc.  Under  these  circumstances,  lengthening 
the  arc  whilst  maintaining  the  current  constant  causes  the 
potential  difference  between  the  carbons  to  increase,  whilst, 
if  the  length  is  kept  constant,  increasing  the  current  causes 
the  potential  difference  between  the  carbons  to  decrease.  If 
the  current  is  increased  beyond  a  certain  point  the  arc 
passes  into  an  unstable  condition,  and  then  finally  begins 
to  hiss.  When  this  is  the  case,  increasing  the  current  over 
wide  limits  hardly  at  all  affects  the  potential  difference  of 
the  carbons. 

It  has  been  shown  by  Mrs.  Ayrton  that  the  primary  cause 
of  hissing  is  the  access  of  air  to  the  crater.  This  occurs 
when  the  crater  becomes  so  large  that  it  overspreads  the 
end  of  the  positive  carbon.  The  same  investigator  has  also 
shown  that  with  open  arcs  there  is  a  sudden  decrease  of 
9  or  10  volts  in  the  potential  difference  of  the  carbons  at 
the  moment  when  the  arc  commences  to  hiss. 

In  every  arc  formed  with  given  carbons  and  for  each  arc 
length  there  is  a  maximum  current  beyond  which  the  arc 
passes  into  the  hissing  state.  In  the  silent  state  it  has 
been  shown  by  Mrs.  Ayrton  that  the  current  for  a  fixed 
length  of  arc  varies  inversely  as  the  excess  of  the  potential 
difference  of  the  carbons  above  a  certain  constant  voltage, 
varying  slightly  with  the  length  of  the  arc.  The  presence 
of  this  constant  term  in  the  expression  for  the  potential 
difference  of  the  carbons,  in  terms  of  the  current  and 
length,  has  given  rise  to  the  theory  that  the  electric  arc 
acts  as  if  it  contained  a  source  of  electric  pressure  which 
operates  against  the  external  electromotive  force  producing 
the  arc.  There  are  various  reasons,  however,  why  this 
position  is  untenable,  and  there  are  also  arguments  supporting 
the  view  that  this  constant  pressure,  which  must  be  first 
applied  before  anv  true  arc  can  be  formed,  represents  one 


ELECTRIC  AEG  LAMPS.  165 

factor  in  the  measurement  of  the  physical  work  that  must 
be  done  in  order  to  volatilise  the  carbon  in  the  crater, 
and  to  maintain  that  production  of  carbon  vapour. 

Carbon  or  any  other  solid  material  can  only  be  made  to 
pass  into  the  gaseous  state  by  imparting  to  it  a  certain 
quantity  of  energy  usually  called  the  latent  heat  of  vapor- 
isation. In  the  case  of  the  electric  arc  the  energy  so  required 
can  only  be  taken  from  the  current  by  creating  a  fall  in 
voltage  or  potential.  Hence,  if  at  the  surface  of  the  positive 
carbon  the  carbon  is  being  boiled  away  into  vapour,  energy 
absorption,  represented  by  a  fall  in  potential,  must  occur  at 
that  place.  Until  this  carbon  vapour  is  manufactured  out  of 
the  solid  carbon  electrode,  there  is  no  possibility  of  the 
production  of  a  true  electric  arc,  consisting,  as  this  does,  in 
the  passage  of  a  current  through  a  column  of  carbon  vapour. 
It  appears  evident,  therefore,  that  before  any  arc  of  finite 
length  can  be  produced  there  must  occur  a  fall  in  voltage. 
Since  the  energy  required  to  volatilise  a  unit  mass  of  carbon 
is  a  constant  quantity,  and  since  the  quantity  of  electricity 
which  can  be  conveyed  by  a  carbon  atom,  molecule  or  ion  is  a 
constant  quantity,  it  would  appear  that  the  absorbed  voltage 
on  this  counter-electromotive  force,  as  it  has  been  called, 
must  also  be  a  constant  quantity.  It  is  this  which  constitutes 
the  35  or  40  volts  required  to  begin  a  carbon  electric  arc. 

In  connection  with  this  question  we  may  note  some 
experiments  by  Victor  von  Lang.  In  his  experiments  he 
formed  electric  arcs  between  poles  of  various  metals,  and 
measured  this  constant  pressure  difference  below  which  no 
arc  of  measurable  length  can  be  maintained.  His  results  are 
seen  in  a  table  on  the  next  page,  and  it  will  be  at  once  noticed 
that  the  order  in  which  the  metals  stand  as  regards  the 
magnitude  of  this  so-called  back  electromotive  force  is  the 
order  of  their  inf  usibility  or  volatility. 


166     ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 


Table  of  Electric  Pressures  or  Voltages  required  to  begin  an  Electric 
Arc  between  poles  of  the  materials  named.  (Victor  von  Lang.) 


Material  used  for  poles  between 
which  the  Arc  is  formed. 

Initial  voltage  for 
an  Arc  between 

production  of 
these  poles. 

Carbon    

35  volts. 
27 
25 
26 
23 
15 
10 

Platinum    

Iron 

Nickel  

Silver 

Cadmium    

If,  then,  the  temperature  of  the  crater  is  the  temperature 
of  the  boiling  metal  or  material  be  ween  which  the  electric 
arc  is  formed,  we  should  expect  such  a  crater  to  have  a 
constant  temperature  and  to  emit  light  of  a  perfectly  con- 
stant composition.  Eeasons  have  accordingly  been  given  by 
different  physicists  for  believing  that,  in  the  carbon  arc,  the 
temperature  of  the  crater  of  the  positive  carbon  has  a  perfectly 
constant  value,  which  is  that  of  the  boiling  point  of  carbon. 

From  the  above  table  of  results  it  will  be  seen  that  the  most 
infusible  material  (carbon)  requires  the  highest  voltage  to  begin 
a  true  electric  arc,  and  generally  that  the  order  of  the  initial 
voltage  is  the  order  of  infusibility  of  the  materials.  It  has 
been  shown  that  the  lower  the  electric  conductivity  of  the 
material  of  which  the  poles  are  formed,  the  greater  is  the 
difference  of  temperature  between  the  positive  and  negative 
poles  when  an  arc  is  produced  between  them.  The  following, 
therefore,  are  the  reasons  why  carbon,  in  its  hard  graphitic 
form,  is  so  peculiarly  suitable  for  use  as  the  poles  or  electrodes 
in  forming  an  electric  arc : — 

1.  It  is  a  rather  poor  conductor  of  electricity  (compared 
with  metals).  Hence  we  obtain  a  great  difference  of  tern- 


ELECTEIC  AEG  LAMPS.  1C? 

perature  between   the  poles ;   in  other  words,  we  gain   the 
advantage  of  having  a  highly-heated  crater  on  one  pole. 

2.  It  is  easily  disintegrated.     Hence  we   obtain  a  longer 
arc  for  a  given  electric  pressure  difference  between  the  poles. 

3.  It  is  very  infusible,  and  its  oxide  is  a  gas ;  and  therefore 
does  not  form  a  deposit  of  oxide  in  and  around  the  arc. 

4.  It  is  an  inferior  conductor  for  heat.     Hence  the  high 
temperature  is  localized  at  the  ends  of  the  poles. 

Just  as  no  material  has  yet  been  found  which  is  superior  to 
carbon  for  the  incandescing  conductor  of  glow  lamps,  so  no 
substance  has  been  yet  discovered  superior  to  carbon  for 
use  in  the  production  of  an  electric  arc. 

Hence  we  must  conclude  that  the  processes  at  work  in  the 
production  of  an  electric  arc  are  somewhat  as  follows  : — In  the 
crater,  carbon  is  being  boiled  into  vapour  at  a  temperature 
probably  about  3,500°  and  4,000°  centigrade.  On  looking 
carefully  at  a  magnified  image  of  the  crater  projected  by  a 
lens  on  to  white  paper,  we  can  see  an  apparent  seething  and 
effervescence  of  the  surface  of  the  intensely  white  hot  floor  of 
the  crater.  This  torrent  of  carbon  vapour  passing  towards 
the  negative  pole  forms  the  violet  core  of  the  arc,  the  violet 
colour  being  that  of  the  vividly-incandescent  carbon  vapour, 
possibly  superheated  to  a  higher  temperature  than  the  boiling 
point  of  carbon  by  the  passage  of  the  electric  discharge  through 
it.  At  the  cooler  negative  pole  some  of  the  carbon  vapour  is 
condensed ;  but  a  portion  of  the  torrent  of  carbon  molecules 
carried  across  is  deflected  back  and  returns  to  the  positive  pole, 
causing  the  golden  aureole  or  flame,  and  creating  thus  a  double 
carbon  current  in  the  arc.  The  negative  carbon  gives  evidence 
after  use  of  having  been  worn  away  by  a  kind  of  sand-blast 
action,  and  is  so  worn  away  into  a  series  of  terraces.  The 
negative  carbon  also  undergoes  a  curious  series  of  changes,  by 
which  a  point  or  knob  of  carbon  is  gradually  formed  on  it, 


168    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

This  knob  or  mushroom  end  finally  breaks  off,  and  is  thrown 
out  of  the  arc.  In  Fig.  61  are  shown  a  series  of  diagrams  illus- 
trating the  gradual  changes  the  negative  carbon  undergoes. 

It  was  shown  by  the  author  in  1890  that,  if  a  third  carbon  pole 
is  dipped  into  the  continuous  current  arc,  so  that  its  tip  is  in 
the  core  of  the  arc  but  not  touching  either  positive  or  negative 
carbon,  or  if  the  electric  arc  is  projected  sideways  by  a  magnet 
on  to  the  third  carbon,  then  a  great  difference  of  electric 
pressure  exists  between  the  positive  or  crater  carbon  and  this 


Stage  1.  Stage  2.  Stage  3. 

FIG.  61. — Diagrams  showing  the  Changes  in  Form  of  the  End  of  the 
Negative  Carbon. 

third  pole,  sufficiently  great  to  illuminate  a  small  incandescent 
lamp  or  to  ring  an  electric  bell ;  but  that  there  is  a  much 
less  difference  of  electric  pressure  between  the  negative  carbon 
and  the  third  pole. 

If  a  continuous  current  electric  arc  is  formed  in  the  usual 
way,  and  if  a  third  insulated  carbon,  held  at  right  angles  to 
the  other  two,  is  placed  so  that  its  tip  just  dips  into  the  arc 
(see  Fig.  62),  we  can  show  a  similar  series  of  experiments  to 
those  described  in  Lecture  II.  under  the  head  of  the 
"Edison  effect."  The  arc  is  rather  more  under  control  if 


**-        OF   TRB 

UNIVERSITY 


ELECTRIC  ARC  LAMPS. 


169 


we  cause  it  to  be  projected  against  the  third  carbon  by  means 
of  a  magnet.  We  have  now  formed  on  the  screen  an  image  of 
the  carbon  poles  and  the  arc  between  them  in  the  usual  way. 
Placing  a  magnet  at  the  back  of  the  arc,  the  flame  of  the  arc 
is  deflected  laterally  and  is  blown  against  a  third  insulated 
carbon  held  in  it.  There  are  three  insulated  wires  attached 
respectively  to  the  positive  and  to  the  negative  carbons  of  the 
arc  and  to  the  third  or  insulated  carbon,  the  end  of  which  dips 
into  the  flame  of  the  arc  projected  sideways  by  the  magnet.  On 
starting  the  arc  this  third  carbon  is  instantly  brought  down  to 
the  same  electrical  potential  as  the  negative  carbon  of  the  arc, 
and  connecting  an  amperemeter  in  between  the  negative  carbon 


FIG.  62. — Electric  Arc  projected  by  magnet  against  a  third  carbon,  and 
showing  strong  electric  current  [flowing  through  a  galvanometer,  G,  con- 
nected between  the  positive  and  third  carbon. 

and  the  third  or  insulated  carbon,  we  get,  as  you  see,  no  indica- 
tion of  a  current.  If,  however,  we  change  the  connections 
and  insert  the  circuit  of  the  galvanometer  between  the  positive 
carbon  of  the  arc  and  the  middle  carbon,  we  find  evidence, 
by  the  violent  impulse  given  to  the  galvanometer,  that  there  is 
a  strong  current  flowing  through  it.  The  direction  of  this 
current  is  equivalent  to  a  flow  of  negative  electricity  from  the 
middle  carbon  through  the  galvanometer  to  the  positive  carbon 
Of  the  arc.  This  shows  that  the  fall  of  potential  between  the 


170    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

carbons  chiefly  takes  place  near  the  positive  carbon.  So 
strong  is  the  current  flowing  in  a  circuit  connecting  the 
middle  carbon  with  the  positive  carbon  that  we  can,  as  you 
see,  ring  an  electric  bell  and  light  a  small  incandescent  lamp 
when  these  electric  current  detectors  are  placed  in  connection 
with  the  positive  and  middle  carbons. 

We  also  find  that  the  flame-like  projection  of  the  arc 
between  the  negative  carbon  possesses  a  unilateral  conductivity. 
Joining  a  small  secondary  battery  of  fifteen  cells  in  series 
with  the  galvanometer,  and  connecting  the  two  between  the 
middle  carbon  and  the  negative  carbon  of  the  arc,  just  as 


FIG.  63. — Galvanometer  G  and  battery  B  inserted  in  series  between 
negative  carbon  of  electric  arc  and  a  third  carbon  to  show  unilateral  con- 
ductivity of  the  arc  between  the  negative  and  third  carbons. 

in  the  analogous  experiment  with  the  incandescent  lamp,  we 
find  we  can  send  negative  electricity  along  the  flame  of  the  arc 
one  way  but  not  the  other.  The  secondary  battery  causes  the 
galvanometer  to  indicate  a  current  flowing  through  it  when 
its  negative  pole  is  in  connection  with  the  negative  carbon  of 
the  arc  (see  Fig.  63),  but  not  when  its  positive  pole  is  in  con- 
nection with  the  negative  carbon.  On  examining  the  third 
or  middle  carbon  after  it  has  been  employed  in  this  way  for 


ELECTRIC  AEG  LAMPS.  171 

some  time,  we  find  that  its  extremity  is  cratered  out  and  con- 
verted into  graphite,  just  as  if  it  had  been  employed  as  the 
positive  carbon  in  forming  an  electric  arc. 

Much  interesting  work  yet  remains  to  be  done  in  studying 
the  physics  of  the  electric  arc,  and  we  cannot  say  yet  that  we 
fully  understand  the  mechanism  which  underlies  this  familiar 
electric  phenomenon. 

We  must  not  pass  on  from  this  part  of  the  subject  with- 
out making  reference  to  experiments  which  have  been  made 
to  determine  the  intrinsic  brightness  of  the  crater  of  the 
electric  arc.  It  has  been  estimated  by  Mr.  A.  P.  Trotter  that 
each  square  millimetre  of  the  incandescent  crater  sends  out 
light  equal  in  brightness  to  170  candles.  Other  observers, 
such  as  M.  Blondel,  have  determined  a  similar  value,  viz., 
160  candles  per  square  millimetre.  That  is  to  say,  an  arc 
light  crater,  having  an  area,  say,  of  30  square  millimetres, 
would  send  out  light  which  would  produce  an  illumination  or 
brightness  on  a  white  surface  placed  parallel  to  it  equal  to 
about  5,000  candles  placed  at  the  same  distance.  The  intrinsic 
brilliancy  of  the  arc  crater  exceeds  that  of  any  other  source  of 
light  with  which  we  are  acquainted,  except  the  sun.  Certain 
materials  exist  for  making  a  comparison  between  solar  tempera- 
ture and  brilliancy,  and  that  of  the  arc  light  crater.  It  is  well 
known  that  the  sun's  edge  (limb)  is  much  less  bright  than  the 
centre  (40  per  cent,  less) ;  but,  as  a  first  rough  approximation, 
we  may  take  it  as  equally  bright,  and  assume  that  the  sun 
radiates  light  as  if  it  were  an  incandescent  circular  disc  of 
852,900  miles  in  diameter.  The  actual  area  of  its  apparent 
surface  is  then  1J  billion  billion  square  millimetres  (I'48'IO24 
sq.  mm.).  Prof.  Young,  in  America,  has  made  an  estimate  of 
the  illuminating  power  of  the  sun,  and  his  conclusion  is  that 
it  is  equal  to  1,575  billion  billion  candles  (1575. 1024  c.p.), 
correction  being  made  for  terrestrial  atmospheric  absorption. 


172    ELEGTEIG  LAMPS  AND  ELECTEIG  LIGHTING. 

Hence  the  mean  intrinsic  brilliancy  of  the  sun's  apparent  surface 
is  about  1,000  candles  per  square  millimetre. 

The  rate  at  which  energy  is  being  sent  out  from  the  sun's 
surface  has  been  estimated  from  data  given  by  Herschell 
and  Pouillet ;  and  more  recently  Forbes  and  Langley  have 
corrected  these  data,  so  that  from  them  Lord  Kelvin 
estimates  the  solar  radiation  as  equal  to  133,000  horse-power 
per  square  metre.  This  corresponds  very  nearly  to  100 
watts  per  square  millimetre.  Accordingly  every  square 
millimetre  of  solar  surface  is  sending  out  energy  as  radiation 
at  a  rate  equal  to  100  watts,  and  producing  a  candle- 
power  of  1,000  candles.  The  sun  is,  therefore,  working  at 
an  "  efficiency,"  as  a  glow-lamp  maker  would  say,  of  T^n  °f  a 
watt  per  candle.  If  we  take  the  mean  value  of  the  estimates 
that  have  been  made  for  the  intrinsic  brilliancy  of  the  crater 
of  the  electric  arc  as  160  candles  per  square  millimetre,  and 
the  rate  of  energy  radiation  as  30  watts  per  square  millimetre, 
which  is  probably  not  far  from  the  truth, *  we  see  that  the 
solar  surface  radiation  is  100  watts,  and  the  solar  intrinsic 
brilliancy  1,000  candles  per  square  millimetre;  whilst  the 
electric  arc  light  crater  has  a  radiation  of  about  30  watts 
and  a  brilliancy  of  160  candles  per  square  millimetre.  The 
brilliancy  of  the  sun  is,  therefore,  according  to  these  figures, 
six  times  that  of  the  electric  arc  crater,  and  ifes  rate  of  sur- 
face radiation  about  three  times.  Compare  this  again  with 
the  incandescent  carbon  of  the  glow  lamp.  A  16-c.p.  glow 
lamp  with  circular  filament  has  a  carbon  filament  five 
inches  long  and  TJotn  °f  an  incn  in  diameter.  The  area  of 

*A  series  of  experiments  made  in  the  author's  laboratory  seemed  to 
show  that  in  various  sized  arcs,  using  from  300  to  1,000  watts,  the  crater 
area  was  always  of  such  size  that  in  every  case  about  27  to  30  watts  was 
dissipated  per  square  millimetre  of  crater  surface.  This  is  on  the  assump- 
tion that  the  whole  work  done  in  the  arc  is  in  the  first  place  expended  in 
the  crater,  and  that  the  rest  of  the  heating  of  the  carbon  is  due  to  secondary 
effects, 


AUG  LAMPS. 


173 


its  apparent  surface  is,  then,  about  30  square  millimetres, 
its  total  surface  is  very  nearly  100  square  millimetres,  and 
it  will  radiate  energy  at  a  rate  equal  to  about  50  watts  when 
giving  a  light  of  16  candle  power.  Hence,  per  square  milli- 
metre of  apparent  surface,  its  intrinsic  brilliancy  is  about 
half  a  candle,  and  its  rate  of  radiation  half  a  watt.  Compar- 
ing the  radiation  power  and  intrinsic  brilliancy  of  the  three 
illuminants — sun,  electric  arc  crater,  and  glow  lamp  filament — 
per  square  millimetre  of  surface,  we  have  a  rough  comparison 
as  follows : — 


— 

Rate  of  radiation  of  energy 
per  square  millimetre 
of  surface. 

Intrinsic  brilliancy  or 
candle-power  per  square 
millimetre. 

Sun  

100  watts. 

1,000  candle-power. 

Electric     Arc 
Crater  
Glow   Lamp 
Filament... 

30      „ 

4      „ 

160 

4 

The  law  connecting  radiation  with  temperature  at  these 
high  temperatures  is  not  yet  definitely  known,  but  the  above 
figures  warrant  the  conclusion  that  the  solar  temperature, 
though  much  higher,  is  not  enormously  higher  than  that  of 
the  crater  of  the  electric  arc.  The  temperature  of  the  sun 
has  sometimes  been  assumed  to  be  millions  of  degrees  Centi- 
grade. All  such  guesses  must  be  exceedingly  wide  of  the  mark. 
The  solar  temperature,  at  least  at  the  surface,  is  probably 
much  nearer  6,000°  or  7,000°C. 

It  is  necessary  to  note  the  proper  manner  of  comparing 
various  illuminating  agents  in  respect  of  their  specific 
energy  radiation  and  light-producing  powers.  If  we  consider 
a  straight  cylindrical  carbon  filament  which  is,  say,  Sin 
long  and  T^th  of  an  inch  in  diameter,  this  filament  will 
have  a  total  surface  of  nearly  Ty^ths  of  a  square  inch, 
but  its  apparent  or  projected  surface  will  only  have  an 


174    ELEGTHIG  LAMPS  AND  ELEGTR1G  LIGHTING. 

area  of  ^th  of  a  square  inch.  Let  this  filament,  when 
incandescent,  give  a  candle-power  of  16  candles  measured 
in  a  direction  at  right  angles  to  itself,  and  absorb  a 
total  power  of  48  watts.  If  we  take  as  our  unit  of  area 
^.^.^th  of  a  square  inch,  called  an  inch-mil,  it  is  clear 
that  the  energy  waste  is  at  the  rate  of  48  watts  for  160 
units  of  surface,  or  1  watt  per  3J  inch-mils.  The  candle- 
power  given  by  the  circular  section  filament  is,  however, 
just  the  same  as  would  be  given  out  by  a  flat  strip 
of  carbon,  kept  at  the  same  temperature,  the  length  of 
which  was  the  same  and  the  width  of  which  was  equal 
to  the  diameter  of  the  round  filament.  Hence  the  brightness 
of  the  surface,  or  candle-power  given  out  perpendicularly 
by  each  unit  of  surface  of  the  round  filament,  is  obtained 
by  dividing  the  whole  candle-power — viz.,  16— by  the  area 
of  the  projected  surface  of  the  circular  filament,  viz.,  ^Vn 
of  a  square  inch.  If,  therefore,  the  unit  of  surface  is  the 
TT5i__th  of  an  inch,  the  brightness  of  the  surface  is  i£ths 
of  a  candle  per  inch-mil,  the  energy  waste  is  at  the  rate 
of  -g-jjths  of  a  watt  per  inch-mil  of  surface. 

In  the  usual  way  of  reckoning  what  is  called  the  "  efficiency  " 
of  the  glow  lamp  it  is  usual  to  divide  the  whole  power 
taken  up  in  the  filament  (in  this  case  48  watts)  by  the 
resultant  or  observed  candle-power  in  one  direction  (in  this 
case  16  c.-p.),  and  to  obtain  a  quotient  (in  this  case  3)  of 
watts  per  candle-power.  If  we  are,  however,  defining  the 
rr.te  at  which  energy  is  dissipated  per  square  unit  of  area 
of  the  incandescent  surface,  and  the  brightness  or  normal 
candle-power  per  square  unit  of  the  surface,  we  see  that 
the  ratio  of  the  numbers  representing  brightness  and  power 
radiated  per  unit  of  area  is  not  the  same  as  the  ratio  of 
the  numbers  representing  the  observed  candle-power  and  the 
total  power  dissipated  when  we  are  dealing  with  glow  lamp 
filaments.  If  we  call  the  total  power  in  watts  radiated 


ELECTRIC  AUC  LAMPS.  175 

per  square  unit  of  area  from  the  incandescent  surface  the 
dissipating  power  of  that  surface,  we  see  that  the  ratio  of 
dissipating  power  to  brightness  is  about  one  watt  per  candle 
for  a  carbon  filament  when  worked  under  the  conditions  of 
temperature  at  which  its  "efficiency"  would  generally  be 
called  three  watts  per  candle-power.  Hence  it  happens  that 
in  the  above  table,  comparing  solar,  arc,  and  glow  lamp 
radiation,  the  brightness  of  the  carbon  filament  is  given 
as  half  a  candle  per  millimetre,  and  the  dissipating  power 
as  half  a  watt  per  millimetre.  It  is  necessary  to  distinguish 
between  the  ratio  of  dissipating  power  to  brightness  and 
total  radiation  of  energy  to  observed  candle-power.  We 
shall  call  the  former  the  watts  per  normal  candle-power 
and  the  latter  the  watts  per  observed  candle-power. 

We  cannot  assume  that  the  law  which  experiment  shows 
connects  together  the  candle-power  and  the  rate  of  energy 
waste  in  watts  in  a  glow-lamp  filament  is  followed  at 
temperatures  much  higher  than  that  at  which  the  incan- 
descent filament  is  usually  worked.  The  temperature  of  the 
carbon  filament  when  being  used  at  about  3  watts  per 
candle-power  is  probably  not  far  from  1,500°C.  or  1,700°C. 
Up  to  that  point  the  candle-power  appears  to  increase 
nearly  as  the  cube  of  the  power  in  watts  taken  up.  We 
cannot,  however,  heat  the  carbon  to  a  temperature  higher 
than  that  which  corresponds  to  one  watt  per  observed 
candle-power  without  rapidly  volatilising  it  in  vacuo.  In 
the  electric  arc  crater  the  temperature  appears  to  correspond 
to  about  one-fifth  of  a  watt  per  normal  candle-power ;  and 
this  temperature  M.  Violle  has  stated  to  be  about  3,600°C., 
or  about  double  that  of  the  incandescent  carbon  of  the  glow 
lamp  at  normal  temperature.  These  estimates  of  arc  crater 
temperature,  however,  probably  err  on  the  side  of  being  too  low, 
and  the  true  crater  temperature  may  be  even  higher  than  the 
above  value.  Stefan,  in  1879,  suggested  that  the  temperature 


176    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

of  a  heated  black  body  and  its  rate  of  radiation  of  energy  were 
connected  as  follows  :  The  fourth  power  of  the  absolute  tem- 
perature of  the  body  varies  as  the  rate  of  radiation  of  energy 
from  a  unit  of  the  surface  of  the  body.  In  other  words,  if  the 
rate  of  radiation  of  energy  is  increased  16  times  the  tempera- 
ture of  the  surface  would  be  doubled,  if  81  times  trebled, 
and  so  on.  It  has  been  asserted  by  several  experiment- 
alists that  this  law  is  inapplicable  to  high  temperatures, 
but  if  we  assume  for  the  moment  that  it  can  in  any  degree 
apply  to  the  range  of  temperature  extending  from  the  glow  lamp 
to  the  solar  surface  temperature,  then,  since  the  dissipating 
powers  of  the  surfaces  of  the  glow  lamp  filament,  arc  crater, 
and  sun,  are,  from  the  table  above,  seen  to  be  nearly  in  the 
ratio  of  1  .'  50  ;  200,  the  absolute  temperatures  should  be 
in  the  ratio  of  1  ;  2J  'm  3f  roughly,  and  this  would  indicate 
the  solar  surface  temperature  as  being  not  much  greater  than 
7,000°C. 

We  must  now  briefly  consider  the  practical  side  of  electric 
arc  lighting.  We  shall  make  no  attempt  to  enter  into  details  as 
to  the  countless  forms  of  arc  lamp  mechanism,  but  in  very 
general  terms,  explain  the  nature  of  that  mechanism.  From 
what  has  been  said  above,  it  will  be  seen  that,  in  order  to 
maintain  an  electric  arc  between  carbon  points,  some  form  of 
apparatus  has  to  be  devised  which  will  automatically  perform 
the  three  following  operations  : — First,  bring  the  carbon  rods 
or  points  together  when  no  current  is  passed  between  them; 
second,  as  soon  as  a  current  passes,  separate  them  to  a 
determined  distance,  or  "strike  the  arc,"  as  it  is  termed  ;  and, 
third,  will  bring  the  carbons  together  slightly  if  the  current 
decreases,  or  separate  them  slightly  if  it  increases.  This  last 
action  is  called  "feeding  the  arc."  In  a  good  arc  lamp 
mechanism  the  "feed"  is  very  smooth  and  uniform,  and  is 
marked  by  an  entire  absence  of  all  jerky  movements.  All 
early  attempts  at  the  invention  of  arc  lamp  mechanism  com- 


ELECTRIC  ARC  LAMPS. 


177 


prised  the  use  of  clockwork,  and  this  involved  many  objec- 
tions. Modern  arc  lamp  mechanism  almost  entirely  depends 
upon  the  employment  of  electromagnets,  which  act  as  the 
source  of  power  to  move  the  carbons.  Without  attempting  any 
extended  description  of  even  a  portion  of  the  devices  employed, 
a  very  general  idea  may  be  obtained  by  considering  the  simplest 
form  of  shunt  and  series  coil  lamps.  One  of  the  carbon  rods, 
say  the  upper  one,  is  fixed  to  an  iron  rod  or  core  I  (see  Fig.  64), 
which  has  one  end  inserted  just  inside  the  hollow  of  a  bobbin 
wound  over  with  thick  insulated  wire,  Se,  and  the  other  end  in- 
serted just  inside  a  bobbin,  S h,  wound  over  with  fine  wire.  The 


FIG.  64. — Diagram  of  Shunt  and  Series  Coil  Arc  Lamp  Mechanism. 
M,  M,  are  the  two  Electric  Supply  Mains  ;  Se,  the  Series  Coil  j  Sh,  the 
Shunt  Coil ;  I,  the  Iron  Core,  and  C,  the  Carbons. 

former  bobbin  is  inserted  in  the  line,  bringing  the  current 
to  the  arc,  and  is  called  the  series  coil.  The  fine  wire  coil  has 
its  wire  joined  across  between  the  two  carbons,  and  is  called 
the  shunt  coil.  The  arrangement  will  be  understood  by 
reference  to  Fig.  64. 

When  the  carbons  are  placed  in  contact,  and  a  current  is 
passed  through  them,  this  current  passes  through  the  series 
coil  on  its  way  to  the  carbons.  It  therefore  energises  that 
coil,  and  causes  it  to  attract  or  pull  in  the  iron  core.  It  will  be 


178    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

understood  by  reference  to  the  figure  that  this  action  immediately 
separates  the  carbons  a  little  way.  At  the  same  time  the 
shunt  coil  is  traversed  by  a  current,  the  action  of  which  is 
to  attract  its  half  of  the  iron  core,  and  therefore  to  exert 
an  influence  drawing  the  carbons  together.  The  carbons 
therefore  become  separated  by  a  small  amount,  thus  striking 
the  arc,  and  the  amount  by  which  they  become  so  separated  is 
determined  by  the  relative  magnetic  power  of  the  two  coils.  If 
the  current  becomes  weakened  by  the  carbons  burning  away, 
then  the  action  of  the  shunt  coil  preponderates,  and  draws  the 
carbons  together  again,  thus  shortening  the  arc  and  increasing 
the  current.  When  the  current  increases  in  strength  the 
series  coil  exerts  the  most  powerful  action  of  the  two,  and 
separates  the  carbons  again  by  a  small  amount.  It  will  be 
seen  that  the  iron  core  is  kept  floating  between  two  balanced 
attractions,  one  of  which  tends  to  draw  the  carbons  attached  to 
it  together,  the  other  to  separate  them.  By  this  simple  means, 
modified  in  practice  by  various  details  of  construction,  an  arc 
lamp  can  be  constructed  which  is  self-feeding  and  which  sets 
itself  in  action  by  the  simple  process  of  switching  it  on  to  the 
circuit. 

There  are  an  immense  variety  of  arc  lamps  depending 
upon  this  principle,  in  all  of  which  various  means  are 
taken  to  make  the  feed  of  the  lamp  properly  gradual.  If  the 
feed  of  the  lamp  is  not  sufficiently  regular  it  shows  itself 
by  rapid  changes  in  the  appearance  of  the  arc.  As  the  arc 
lengthens  the  violet  light  of  the  true  arc  preponderates,  and 
the  lamp  burns  with  a  much  more  bluish  colour.  If  the  carbons 
then  come  together  suddenly  the  lamp  is  very  liable  to  hiss 
and  to  burn  for  some  time  with  a  more  reddish  light  until 
the  carbons  are  separated.  In  lamps  with  imperfect  feeding 
mechanism  these  changes  give  rise  to  irregularities  in  the 
light  which  are  very  annoying.  In  a  good  arc  lamp  the  feeding 
mechanism  should  work  with  such  perfect  regularity  that  the 


ELECTRIC  ARC  LAMPS.  179 

movement  of  the  carbons  can  hardly  be  discerned  with  a 
magnifying  glass.  The  best  manner  of  examining  the  feed  of 
an  arc  lamp  is  to  project,  by  means  of  a  lens,  an  image  of  the 
arc  lamp  upon  the  screen,  and  then  to  examine  this  optical 
image.  The  motion  produced  by  the  feeding  mechanism 
is  then  magnified  in  the  same  ratio  as  the  dimensions  of  the 
arc,  and  any  irregularity  of  motion  is  very  easily  detected. 

One  word  ought  to  be  said  with  regard  to  the  quality  of 
carbons  used  in  arc  lighting.  A  vast  amount  of  ingenuity 
and  capital  has  been  expended  in  investigations  intended 
to  discover  the  best  processes  for  the  manufacture  of  arc 
light  carbons.  These  carbons  are  now  made  as  above 
mentioned  by  forming  into  a  paste  some  very  pure  form  of 
carbon  by  means  of  a  fluid,  such  as  tar  or  syrup,  capable 
of  being  carbonised.  This  paste  is  squeezed  out  into  rods  by 
powerful  hydraulic  presses,  and  these  rods  are  then  dried  and 
baked.  Carbons  are  classified  as  cored  or  non- cored  carbons. 
In  the  cored  carbons  the  centre  of  the  carbon  rod  is  perforated 
by  a  longitudinal  hole,  thus  forming  a  carbon  tube,  and  this 
carbon  tube  is  filled  up  with  a  less  dense  form  of  carbon. 
These  cored  carbons  are  generally  used  for  the  positive  carbon 
of  the  arc,  and  the  object  of  making  a  softer  central  portion  to 
the  carbon  is  to  determine  the  position  of  the  crater  and 
compel  it  to  keep  a  central  position. 

As  already  explained,  in  operating  continuous  current  arc 
lamps  the  positive  or  crater  carbon  is  generally  placed  upper- 
most. Under  these  circumstances  there  is  a  cone  of  shadow 
directly  under  the  lamp  which  is  due  to  the  negative  carbon. 
Mr.  Crompton  has  found  it  to  be  a  great  advantage  to  use 
very  much  smaller  negative  carbons  than  is  usually  done,  and 
in  his  arc  lamp  he  uses  the  smallest  negative  carbon  which 
can  be  employed  practically.  On  the  other  hand,  a  too 
small  negative  carbon  is  liable  to  become  heated  over  a  greater 

N2 


180    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

portion  of  its  length,  and  therefore  a  practical  limit  is  placed 
to  the  use  of  very  small  negative  carbons. 

The  quality  of  carbons  used  and  their  form  have  a  great 
effect  upon  the  performance  of  an  arc  made  with  them. 
Violent  movements  of  the  arc  and  periods  of  hissing  or 
humming  are  very  objectionable  in  the  case  of  electric  arcs 
used  for  illuminating  purposes.  These  undesirable  actions 
are  under  control  to  a  large  extent,  and  depend  much  upon 
the  quality  of  carbons  used.  In  the  arc  lamps  used  for 
illuminating  purposes  it  is  generally  usual  to  employ  a 
positive  or  upper  carbon  about  14  or  15  millimetres  in 
diameter,  and  a  negative  or  lower  carbon  about  10  or  11 
millimetres  in  diameter.  If  the  arc  is  a  10-ampere  arc  the 
current  density  is  then  about  50  amperes  per  square  inch 
of  carbon  section.  If  the  current  density  is  too  great  the 
crater  will  be  too  large  and  will  extend  over  the  edge  of  the 
positive  carbon.  Access  of  air  will  then  cause  the  arc  to 
begin  hissing.  At  the  moment  when  hissing  begins  the 
difference  of  pressure  between  the  carbons  falls  and  the 
current  increases.  The  true  arc  then  loses  its  transparency 
and  becomes  transformed  into  an  incandescent  mist  which 
hides  the  crater.  When  hissing  ceases  the  arc  again  becomes 
violet  and  transparent,  and  the  surface  of  the  crater  appears 
to  be  covered  with  black  specks  which  gradually  disappear. 
Whilst  hissing  lasts  the  brightness  of  the  crater  is  diminished 
and  the  violet  colour  of  the  arc  becomes  replaced  by  a  very 
characteristic  blue  green,  which  seems  to  point  to  a  lowering 
of  the  temperature. 

It  is  usual  to  use  an  upper  or  crater  carbon  which  is  cored, 
that  is  to  say,  has  a  longitudinal  hole  through  it  filled  up 
with  a  softer  carbon.  This  has  the  effect  of  maintaining  the 
crater  in  a  central  position  and  preventing  the  wandering 
motion  of  the  crater,  which  is  in  many  ways  objectionable. 


ELECTRIC  ARC  LAMPS.  181 

Numerous  experiments  have  been  tried,  by  using  carbons 
cored  with  various  materials  or  saturated  with  various  salts, 
to  improve  the  quality  of  the  light.  On  the  whole,  however, 
the  most  satisfactory  result  is  obtained  by  the  use  of  the 
piirest  carbons. 

In  an  ordinary  10-ampere  arc  lamp,  as  used  for  public 
lighting,  the  carbons  employed  each  burn  away  at  the  rate  of 
about  one  inch  per  hour.  This  disappearance  of  the  carbon 
is  largely  caused  by  actual  combustion  of  the  carbon  by  the 
air.  A  lamp  of  this  type  used  all  night,  every  night  of  the 
year  from  dusk  to  dawn,  consumes  about  600ft.  of  carbons  per 
annum.  If  the  positive  carbon  is  an  18  millimetre  carbon 
and  the  negative  a  12  millimetre  the  consumption  of  each 
size  of  carbon  is  about  70f  fc.  per  thousand  hours  of  burning, 
including  wastage. 

At  present  prices  the  carbons  for  1,000  hours  of  burning 
cost,  wholesale,  about  15s.  Hence,  if  an  arc  lamp  using  such 
carbons  is  burnt  from  dusk  to  midnight  every  night  the  cost 
of  carbons  is  about  £1.  10s.  per  annum.  These  carbons  are 
sold  in  lengths  of  15in.  or  18in.,  and  at  regular  intervals  an 
arc  lamp  requires  to  be  trimmed  or  re-carboned.  In  the  case 
of  public  arc  lighting  one  man  and  one  boy  can  trim  about 
60  lamps  per  day  unless  the  weather  is  very  bad.  The  cost 
of  the  labour  per  lamp  is  about  £2  per  annum.  Hence,  for 
labour  of  trimming  and  carbons  the  cost  of  maintenance  of  a 
public  arc  lamp  is  about  £3.  10s.  to  £4  per  annum. 

If  the  lamp  is  a  10-ampere  lamp  it  will  use  500  watts  per 
hour  or  one  Board  of  Trade  unit  in  every  two  hours  of 
burning.  If  arc  lamps  are  lighted  at  dusk  and  burn  until 
midnight  the  annual  hours  of  usage  are  1,800  to  2,000.  A 
600- watt  arc  will,  therefore,  use  900  to  1,000  Board  of  Trade 
units  per  annum,  and  if  this  energy  is  supplied,  say,  at  3d. 


182   ELECTRIC  LAMPS  AND  ELECTJRIC  LIGHTING. 


per  unit,  the  annual  cost  of  electric  energy  for  a  500-watt  arc 
lamp  burnt  from  dusk  to  midnight  will  be  £11  to  £12,  and, 
including  carbons,  labour  for  trimming  and  repairs,  will  cost 
£15  to  £16  per  annum  to  maintain.  Street  arc  lamps  are 
always  mounted  on  high  standards,  20ft.  to  25ft.  above  the 
roadway.  These  are  placed  from  50  to  100  yards  apart. 
The  following  table  gives  some  data  as  to  the  distance, 
hours  used,  height  above  roadway  and  annual  price  charged 
for  arc  lamps  in  various  towns  : — 


Town. 

Distance 
apart  of 
arc  lamps 
in  yards. 

Total 
number  of 
hours'  use 
in  year. 

Price 
charged  per 
arc  per 
annum. 

Height  of 
arc  above 
roadway 
in  feet. 

Current 
in 
amperes. 

Blackpool    ... 

50 
70 

2,000 
3  864 

£    s.    d. 

25 

10 

Bristol  

50 

22 

10 

Cardiff  

5570 

3,824 

18    0    0 

Chelmsford  ... 

80 
45-90 

3,687 
3  500 

22  10    0 

— 

— 

Hanley  

50-140 

3,900 

16    0    0 



Kingston  
Newport  
Portsmouth... 

110 
70 
90-100 

3,400 

2,787 
1,800 

20    0    0 
25    0    0 
18    0    0 

25 

12 

In  making  calculations  as  to  the  cost  of  running  arc  lamps 
it  is  useful  to  have  at  hand  the  total  hours  of  burning  per 
annum  when  the  lamps  are  lighted  and  extinguished  at 
certain  hours  as  follows  : — 


Hours  of  lighting. 

Annual  hours  of  burning. 

4  374  hours 

Sunset  to  11  p  m  

1,821     „ 

Sunset  to  midnight  

2,186     „ 

Midnight  to  sunrise        .        

2,188     „ 

11  p  m   to  sunrise   

2,553     „ 

In  the  employment  of  arc  lamps  for  illuminating  purposes, 
owing  to  the  fact  that  the  light  is  radiated  from  an  exceed- 


ELECTHIC  ARC  LAMPS. 


183 


ingly  small  area  (in  a  300-watt  lamp,  about  ^jth  part  of  a 
square  inch),  the  light  sent  out  by  the  lamp  throws  very 
sharply  defined  shadows  if  the  lamp  is  used  without  a  shade. 
Hence  it  is  usual  to  enclose  the  lamp  in  a  large  semi-opaque 
globe.  These  globes  cut  off  from  40  to  60  per  cent,  of  the 
light  in  various  directions ;  but,  by  presenting  a  larger  light- 
giving  area  they  cast  less  well-marked  shadows  and  are  less 
dazzling  to  the  eye.  On  the  other  hand,  this  wasteful  absorp- 
tion of  the  light  causes  the  lamp  to  be  less  efficient  as  an 
illuminating  agent.  In  order  to  meet  this  difficulty  the 
device  is  frequently  adopted  of  employing  inverted  arc  lamps. 
In  this  arrangement  the  arc  lamp  is  hung  from  the 


FIG.  65. — Inverted  Arc  Lamp  and  Shade  for  Workshop  and  Factory 
Lighting.     The  dotted  line  shows  the  place  of  the  Conical  Reflector. 

ceiling  or  roof  of  the  building  to  be  illuminated  with  the 
negative  carbons  uppermost  (see  Fig.  65),  and  the  positive  or 
crater  carbon  is  placed  at  the  bottom  of  the  pair.  The  lamp 
is  surrounded  with  a  conical  shade,  the  purpose  of  which  is  to 
throw  the  light  up,  and  in  addition  sometimes  a  white  umbrella- 
like  shade  is  suspended  over  the  lamp,  from  which  the  light  is 
again  reflected  downwards.  A  room  illuminated  by  these 
inverted  arc  lamps  is  therefore  flooded  with  light,  none  of  the 


184     ELEGTRIG  LAMPS  AND  ELECTRIC  LIGHTING. 

rays  of  which  come  directly  from  the  arc,  but  only  after 
reflection  from  the  hood  or  the  ceiling.  A  very  pleasing, 
diffused  light  is  thus  produced,  and  inverted  arc  lighting  has 
been  tried,  in  many  cases  with  great  success  in  workshops, 
dye  works  and  factories.  You  are  able  to  judge  of  the  effect 
of  it  yourselves  if  this  theatre  is  illuminated  for  a  few  minutes 
by  means  of  a  couple  of  inverted  arc  lamps  which  have  been 
lent  to  me  by  Messrs.  Parsons  and  Co.  If  a  workshop  is 
illuminated  by  arc  lamps  in  the  ordinary  manner,  even 
although  they  are  protected  by  semi-opaque  globes,  the  light 
casts  sharp  shadows  around  and  under  the  tools,  and  the 
workmen  are  therefore  sometimes  unable  to  get  the  light 
necessary  for  their  work ;  but  the  inverted  arc  lighting  does 
away  with  this  difficulty,  and  in  places  so  illuminated  there 
are  practically  no  shadows  at  all. 

The  necessity  of  re-carbonising  arc  lamps  used  for  public 
lighting  every  day  has  turned  the  stream  of  invention  of 
late  years  in  the  direction  of  improvements  to  obviate  this 
necessity,  with  the  result  that  we  now  possess  very  efficient 
enclosed  arc  lamps  which  will  burn  with  one  pair  of  carbons 
for  150  or  200  hours.  It  has  already  been  explained  that 
the  consumption  of  tho  carbon  rods  is  largely  due  to  the 
actual  combustion  by  the  air  of  the  incandescent  tip  of  the 
carbons.  The  problem  is  to  prevent  this  combustion.  At 
first  sight  nothing  would  seem  more  simple,  the  apparently 
obvious  solution  of  the  difficulty  being  to  enclose  the  arc  in 
an  air-tight  glass  globe.  This  device  was  tried  very  early 
in  the  history  of  electric  lighting.  The  result  of  using  a 
completely  air-tight  globe  is  that  the  contained  oxygen  is 
removed  by  combustion  with  the  carbon,  and  some  of  the 
carbon  vapour  forming  the  arc  diffuses  through  the  vessel 
and  is  deposited  on  the  cool  sides  of  the  containing  vessel 
as  a  black  deposit.  The  transparency  is  thereby  affected. 
This  always  ensues  if  the  containing  vessel  is  perfectly 


ELECTRIC  ARC  LAMPS. 


185 


enclosed.     It   was,   however,   shown  by   Marks  that   if  the 
vessel  was  not  completely  closed  it  was  possible  to  admit 


H  MTS  h 


HANGER 


SWITCH 


RESISTANCE  COIL- 


MAGNETS 


CLUTCH-- 


CARBON  ROD 


UPPER  CARBON- 


GAS  CHECK  PLUG- 
INCLOSING  BULB 

LOWER  CARBON 


LOWER  CARBON  HOLDER 


HOOK  FOR  TAIL  PIECE 

Fio.  65A.— The  Marks  High  Potential  Enclosed  Arc  Lamp. 

air  in  such  a  limited  manner  that  it  destroyed  by  combustion 
the  carbon  vapour  escaping  from  the  arc,  but  was  not 
admitted  freely  enough  to  cause  very  wasteful  oxidation  of 


186      ELECTRIC  LAMPS  AND  ELECTRIG  LIGHTING. 

the  carbon  rods  themselves.  An  arc  lamp  so  enclosed  in 
a  glass  receptacle,  with  a  limited  access  of  air  to  the 
arc,  is  called  an  enclosed  arc  lamp.  In  Fig.  65A  is  shown 
an  enclosed  arc  lamp  The  top  of  the  enclosing  bulb  is 
closed  by  a  gas  check  plug  or  plate  (see  Fig.  GOB),  which 


FIG.  65B.— Enclosure  with  Gas- Check  Plug. 

admits,  through  a  small  hole,  a  limited  supply  of  air  to 
the  arc,  sufficient  to  consume  the  volatilised  carbon,  yet  not 
sufficient  to  cause  very  great  consumption  of  the  carbon  rods 
themselves. 

The  marked  peculiarity  of  the  enclosed  arc  lamp  is  that 
the  carbons  do  not  burn  to  a  crater  on  the  positive,  and  a 


ELECTRIC  ARC  LAMPS. 


187 


sharp  tip  or  mushroom  on  the  negative,  but  they  maintain 
a  nearly  flat  surface  (see  Fig.  65c).  This  feature  affects 
the  distribution  of  light.  The  illuminating  curve  of  the 
enclosed  arc  is  not  characterised  by  such  a  strongly  marked 
maximum  value,  as  is  that  of  the  open  arc.  We  have 
already  seen  that  the  open  arc  sends  out  rays  in  a  direction 
enclosed  at  about  60°  below  the  horizon,  which  have 
generally  three  times  or  more  the  intensity  of  the  horizontal 
ray.  Also  there  is  a  very  great  difference  between  the 


Open  Arc  :  6  amperes,  45  volts.       Enclosed  Arc  :  6  ampere-,  45  volts. 
FIG.  65c. 

mean  spherical  candle  power  of  an  open  arc  lamp  and  its 
maximum  candle  power.  It  is  for  this  reason,  amongst 
others,  that  all  such  terms  as  "an  arc  lamp  of  2,000 
candle  power,"  are  perfectly  meaningless.  In  the  case  of 
the  enclosed  arc  lamps,  there  is  not,  however,  such  a  great 
difference  between  the  mean  spherical  and  maximum  illu- 
minating power.  The  enclosed  arc  smits  its  rays  of  greatest 
illuminating  power  more  in  a  horizontal  direction. 


188  ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

Summarising  the  advantages  of  the  enclosed  arc,  they 
are  as  follows  : — 

(1.)  The  cost  of  carbons  per  hour  is  about  ^th  of  that 
for  the  open  arc. 

(2.)  The  cost  of  attendance  is  also  far  less. 

(3.)  There  is  a  more  uniform  distribution  of  light  from 
the  arc. 

Against  these  advantages  must  be  set : — 

(1.)  The  cost  of  renewals  of  the  inner  glass  vessel  which 
occasionally  breaks. 

(2.)  The  lower  resultant  candle  power  per  watt,  due  to 
the  absorption  of  light  by  the  inner  glass  vessel  and  outer 
globe,  which  is  also  necessary  to  diffuse  the  light. 

It  is  found  by  experience  that  if  the  inner  and  outer 
globes  are  both  of  opalescent  glass,  that  the  total  loss  by 
absorption  of  light  is  more  than  50  per  cent.  The  best 
results  are  obtained  by  using  an  inner  opalescent  vessel  and 
a  clear  glass  outer  globe. 

The  enclosed  arc  lamps  as  now  made  burn  without  atten- 
tion for  150  or  200  hours  on  100-volt  circuits.  Small  arc 
lamps  are  made  taking  3,  4  and  6  amperes,  and  are  very 
convenient  for  interior  illumination.  For  indoor  use  it  is 
essential  to  enclose  the  inner  vessel  in  a  glass  spherical 
globe,  which  may  be  of  opalescent  glass,  to  soften  and 
diffuse  the  light,  but  for  outdoor  use  it  is  best  to  employ 
an  opalescent  inner  vessel,  and  a  clear  outer  globe.  Enclosed 
arc  lamps  are  now  also  made  to  burn  on  200  volt  circuits. 

Enclosed  arc  lamps  are  constructed  for  use  with  alter- 
nating as  well  as  continuous  currents.  Generally  speaking, 
however,  the  alternating  arc  lamp  has  not  the  same  efficiency 
as  the  continuous-current  arc  lamp — that  is  to  say,  for  the 


OK   THK 

UNIVERSITY 


ELECTRIC  ARC  LAMPS. 


same  moan  spherical  candle  power  the  alternating-current 
arc  lamp  will  require  a  greater  expenditure  on  it  of  electric 
power.  Great  attention  has  been  paid  of  late  years  to  the 
phenomena  of  the  alternating-current  arc.  If  an  arc  is 
formed  between  carbon  poles  with  an  electric  current  which 
is  not  continuous,  but  which  changes  its  direction  or  alter- 
nates 100  or  200  times  a  second,  then  an  arc  is  formed 
which  is  called  an  alternating-current  arc.  The  electric 


P.D. dynamo 
Current 
P.D.  arc. 


.  .D. dynamo 
Current 
P.D.  arc.     . 


FIG.  65o. 

arc  has  a  certain  persistence,  so  that  the  current  can  be 
interrupted  for  a  very  small  fraction  of  a  second,  without 
extinguishing  the  arc.  If,  then,  the  current  alternates,  each 
carbon  in  turn  becomes  the  crater  carbon,  and  during  the 
short  period  of  time  it  is  the  positive  or  crater  carbon,  it 
emits  more  light  than  when  it  is  the  negative  carbon.  If 
an  alternating  arc  is  examined  through  slits  in  a  revolving 
disc  which  is  driven  in  step  with  the  alternations  of  the 


190      ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

arc,  it  is  possible  to  examine  the  various  phases  of  light- 
radiating  power  passed  through  by  each  carbon.  It  is  then 
seen  that  there  is  a  periodicity  or  fluctuation  in  the  light- 
giving  power  of  each  carbon.  This  variation  in  the  light 
given  out  by  each  carbon  follows  the  variation  of  current 
through  the  arc.  The  arc  has,  however,  a  peculiar  power  of 
re-acting  upon  the  impressed  potential  difference  or  external 
electromotive  force.  If  the  machine  (called  an  alternator) 
which  supplies  the  current  is  such  that  its  electromotive 
force  varies  periodically  in  the  manner  represented  by  the 
ordinates  of  the  dotted  curve  shown  in  Fig.  65D,  then  the 
potential  difference  and  the  current  through  the  arc  may,  and 
generally  will,  vary  in  a  totally  different  manner,  as  shown 
by  the  broken  and  firm  line  curves  in  the  same  diagrams.  It 
has  been  shown  that  under  some  conditions  the  arc  formed 
between  a  metal  pole  and  a  carbon  pole  furnishes  a  current 
only  in  one  direction,  although  the  impressed  electromotive 
force  is  an  alternating  arc.* 

We  cannot,  in  conclusion,  do  more  than  make  brief  allusion 
to  the  manner  in  which  arc  lamps  are  employed  in  practical 
work.  One  method  is  by  the  employment  of  arc  lamps  arranged 
in  series.  In  this  arrangement  the  arc  lamps  to  the  number  of 
80  or  50  are  placed  on  one  circuit,  so  that  the  same  electric 
current,  say  one  of  ten  amperes,  flows  through  each  lamp  in 
turn.  As  each  lamp  requires  to  have  about  50  volts  on 
the  terminals  in  order  to  make  it  work  properly,  it  will  be 
seen  that  the  arrangement  necessitates  the  use  of  a  dynamo 
machine  giving  a  very  high  electric  pressure — from  1,500 

*  The  reader,  desirous  of  more  advanced  information,  as  to  the  alter- 
nating-current arc,  may  be  referred  to  the  following  original  Papers  : 
"  Experiments  on  Alternating-Current  Arcs  by  the  aid  of  Oscillographs," 
by  W.  Duddell,  E.  W.  Marchant,  Journal  of  the  Institution  of  Electrical 
Engineers,  Vol.  XXVIII.,  1899,  or  The  Electrician,  Vol.  XLIL,  p.  857. 
Also  "An  Analytical  Study  of  the  Alternating-Current  Arc,"  by  J.  A 
Fleming  and  J.  E.  Petavel ;  Phil.  Mag.,  April,  1896,  p.  315. 


ELECTRIC  ARC  LAMPS.  191 

to  3,000  volts.  Series  arc  lighting  is  employed  to  a  con- 
siderable extent  in  street  lighting,  for  the  reason  that  the 
conductors  conveying  the  current  can  be,  comparatively 
speaking,  small  and  inexpensive.  Arc  lamps  may,  however,  be 
arranged  in  parallel  instead  of  in  series.  In  this  case  two 
or  more  lamps  are  placed  together  across  the  mains  between 
which  a  constant  electric  pressure  is  being  maintained. 
Suppose,  for  instance,  in  any  district  an  electric  lighting 
supply  company  has  the  mains  laid  down  for  giving  a  supply 
at  110  volts  for  incandescent  lighting,  then  two  arc  lamps  can 
be  placed  across  these  mains  in  series,  a  small  resistance  being 
added  in  order  to  control  the  flow  of  current  through  the 
lamps.  At  the  same  time  any  number  of  these  pairs  can  be 
arrranged  across  between  the  mains  like  the  rungs  of  a  ladder. 
In  some  cases  five  to  ten  arc  lamps  are  worked  in  series  across 
mains  having  a  difference  of  potential  between  them  of  from 
250  to  500  volts,  a  number  of  these  series  of  five  or  ten  being 
strung  across  in  parallel  between  two  supply  mains.  An  ob- 
jection which  has  always  been  urged  against  series  arc  lighting 
is  that  it  necessitates  the  employment  of  high  pressure  currents, 
which  are,  of  course,  relatively  much  more  dangerous  than 
low  pressure  currents.  In  series  arc  lighting  each  lamp  has, 
moreover,  to  be  provided  with  an  apparatus  called  an  automatic 
cut-out,  so  that  if  the  lamp  becomes  extinguished  by  the 
carbons  being  separated  too  far  or  burning  out,  the  cut-out 
closes  the  circuit,  and  does  not  permit  the  current  which  is 
supplying  the  other  lamps  in  the  series  to  be  interrupted. 
In  the  case  of  alternating-current  arc  lamps,  they  may  either 
be  run  in  series  on  a  high  tension  alternating-current  circuit, 
as  is  done  in  the  electric  lighting  of  Kome,  or  each  lamp  may 
be  provided  with  a  small  transformer,  so  that  current  can  be 
taken  from  high  tension  mains  and  reduced  down  to  a 
pressure  at  which  it  is  convenient  to  work  the  arc  lamps. 
We  shall  in  the  next  Lecture  proceed  to  explain  more  in  detail 
the  nature  of  this  pressure-reducing  device. 


LECTURE  IV. 


THE  Generation  and  Distribution  of  Electric  Current. — The  Magnetic 
Action  of  an  Electric  Current.— The  Magnetic  Field  of  a  Spiral 
Current. — The  Induction  of  Electric  Currents. — The  Peculiar 
Magnetic  Property  of  Iron. — Iron  and  Air  Magnetic  Circuits. — The 
typical  cases  of  an  Iron  Circuit  with  and  without  Air  Gaps. — The 
prototypical  forms  of  Dynamo  and  Transformer. — The  Transformation 
of  Electrical  Eoergy. — Hydraulic  Illustrations. — The  Mechanical 
Analogue  of  a  Transformer. — The  Mode  of  Construction  of  an 
Alternate  Current  Transformer. — The  Fundamental  Principle  of  all 
Dynamo  Electric  Machines. — Alternating  and  Direct  Current  Dynamos. 
— Alternating  Current  Systems  of  Electrical  Distribution. — Descrip- 
tion of  the  Electric  Lighting  Station  in  Rome. — The  Tivoli-Rome 
Electric  Transmission. — Views  of  the  Tivoli  Station. — Continuous 
Current  Systems. — The  Three-Wire  System. — Description  of  St. 
Pancras  Vestry  Electric  Lighting  Station.— 
Liverpool,  Glasgow,  and  Brussels  Electric 
Lighting  Stations.— Direct-driven  Dynamos. 
— Alternating  and  Continuous  Current 
Systems  Contrasted.— Secondary  Batteries. 
—  House  Meters.  —  Maximum  Demand 
Meters. — Conclusion. 

N  this  fourth  and  last  Lecture  it  will 
be  necessary  to  direct  our  atten- 
tion very  briefly  to  the  subject  of 
the  generation  and  distribution  of 
electric  current  for  the  purposes 
of  electric  illumination.  To  en- 
deavour to  cover  anything  but  a 
small  portion  of  this  immense 
subject  in  the  space  of  one  lecture 

would  be   to  confuse  and  bewilder  rather  than   to  inform. 

It  will,   therefore,  be  necessary  to  limit  our  discussion  to 


194    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

certain  narrow  lines,  and  not  to  attempt,  in  any  way,  to 
give  even  an  outline  of  the  whole  of  the  methods  which  are 
at  the  present  moment  employed  for  this  purpose.  Avoiding 
technicalities,  however,  the  endeavour  will  be  made  to  enable 
the  reader  to  grasp  certain  broad  general  principles  which 
underlie  the  construction  of  all  the  appliances  used  for  the 
generation  and  distribution  of  electric  current  for  industrial 
purposes,  and  then  we  shall  proceed  to  describe  generally 
the  arrangements  employed  in  the  electric  lighting  of  a 
modern  city.  We  must  return,  in  the  first  place,  to  facts 
touched  upon  in  the  First  Lecture,  and  examine  more  in 
detail  the  relation  of  magnetism  and  electric  currents.  In  so 
doing  it  may  be  an  advantage,  for  the  sake  of  brevity  and 
clearness,  to  depart  a  little  from  the  usual  phraseology.  We 
must  also  trace  our  way  in  the  light  of  a  series  of  pre- 
liminary experiments. 

It  has  already  been  pointed  out  that  a  wire,  conveying  what 
we  call  an  electric  current,  exerts  a  magnetic  influence,  and 
at  the  same  time  becomes  heated.  In  fact,  the  phrase,  "an 
electric  current,"  is  merely  a  comprehensive  expression  used 
to  denote  all  the  effects  of  heat  and  magnetism  that  are  known 
to  exist  in  and  around  a  wire  which  we  speak  of  as  being 
traversed  by  an  electric  current.  If  we  take  a  conducting  wire 
— say  a  copper  wire — and  pass  through  it  a  strong  electric 
current,  we  find  that  such  a  wire,  when  dipped  into  iron 
filings,  takes  them  up,  the  filings  clinging  to  the  wire  forming 
a  bunch  around  it.  This  fact  was  discovered  about  1820  by 
the  French  experimentalist  Arago.  We  can  more  carefully 
and  deliberately  explore  the  nature  of  this  magnetic  action 
of  a  wire  conveying  a  current  in  the  following  way  :  Passing 
the  copper  wire  through  a  hole  in  a  glass  plate,  or  card,  so 
that  the  wire  stands  vertically  to  the  plate,  we  then  sprinkle 
the  glass  plate  uniformly  with  steel  filings,  and,  by  causing  a 
powerful  current  to  pass  through  the  copper  wire,  and  by  gently 


ELECTRIC  DISTRIBUTION. 


195 


tapping  the  glass  plate,  the  filings  arrange  themselves  in 
a  series  of  concentric  circular  lines  round  the  wire  (see 
Fig.  66).  The  glass  plate  being  placed  in  this  vertical 
lantern,  the  image  of  these  circular  lines  of  filings  is  projected 
upon  the  screen.  Taking  a  very  small  magnetised  compass 
needle,  and  holding  it  in  any  position  near  the  glass  plate, 
it  at  once  becomes  evident  that  the  little  magnetised  needle 
always  places  itself  in  the  same  direction  as  the  circular 
lines  mapped  out  by  the  iron  filings.  This  fact  may  be 


FIG.  66.— Curves  formed  by  sprinkling  Iron  Filings  on  a  Card  C,  per- 
forated by  a  Wire  conveying  an  Electric  Current,  thus  showing  the  lines  of 
Circular  Magnetism  round  the  Conductor  A  B. 


viewed,  in  the  following  light :  The  circular  magnetisa- 
tion of  the  space  round  the  wire  is  a  manifestation  of 
a  part  of  the  properties  of  the  electric  current,  and  may 
properly  be  called  the  magnetic  part  of  the  current, 
and  is  called  the  magnetic  flux  surrounding  the  con- 
ductor. In  fact,  the  magnetic  flux  is  a  part  of  that 
which  we  call  the  electric  current.  The  direction  of  this 
magnetic  flux  round  a  conductor  may  be  ascertained  at 
any  place  by  the  use  of  iron  filings  as  above  described. 


196     ELECTEIC  LAMPS  AND  ELECTRIC  LIGHTING. 

A  very  little  examination  of  the  distribution  of  the  iron  filings 
shows  us  that  they  lie  closer  together  at  points  near  to  the 
wire  than  at  points  far  away. 

It  is  important  to  examine  another  instance  of  the  same 
kind.  Here  is  a  large  bobbin  of  wire,  the  wire  being  covered 
over  with  cotton  and  wound  on  a  paper  tube.  Into  the 
interior  of  this  bobbin  is  placed  a  slip  of  glass  on  which  steel 
filings  have  been  sprinkled.  On  passing  an  electric  current 
through  the  wire  and  tapping  the  glass,  we  find  that  the  steel 
filings  arrange  themselves  inside  the  bobbin  in  a  series  of 
nearly  parallel  straight  lines  (see  Fig.  67),  and  that,  in  this 
particular  case,  it  is,  therefore,  evident  that  the  electric  current, 


Fio.  67. — Curves  formed  by  Iron  Filings  sprinkled  on  a  Card,  through 
holes  in  which  a  spiral  wire  is  laced,  showing  the  lines  of  Magnetic  Flux 
or  Induction  linked  with  a  Spiral  Electric  Current. 

flowing  through  the  wire  in  a  series  of  nearly  circular  turns, 
is  accompanied  by  a  distribution  of  magnetism,  or  a  magnetic 
flux,  in  a  direction  along  the  axis  of  the  bobbin.  By  exploring 
the  space  outside  the  bobbin  with  a  magnetic  compass  needle 
it  is  very  easy  to  show  that  the  magnetic  flux  which  flows 
in  one  direction  in  the  interior  of  the  bobbin  flows  back 
along  the  outside  of  the  bobbin  in  an  opposite  direction, 
and  so  completes  what  is  called  a  magnetic  circuit.  What- 
ever particular  form  we  give  to  the  conductor  conveying 
the  electric  current,  we  always  find  that  there  is  a  magnetic 
flux  in  the  space  round  it  along  a  magnetic  circuit  which 
is  linked  with  the  conducting  circuit,  This  co-  linking 


ELECTRIC  DISTRIBUTION.  197 

of  electric  conducting  circuits  and  magnetic  circuits  is  the 
fundamental  fact  of  electro-magnetism. 

So  far  the  cases  we  have  been  examining  have  been  such 
that  the  magnetic  flux  exists  in  a  circuit  which  is  wholly 
occupied  by  air  as  a  medium.  37,  instead  of  permitting  the 
magnetic  flux  to  take  place  in  air,  it  takes  place  in  iron 
wholly  or  partly,  then  it  is  found  that  the  same  electric 
current  flowing  in  the  wire  would  produce  a  much  more 
powerful  magnetic  flux  in  the  iron  circuit  than  it  does  in  the 
air  circuit. 

It  is  necessary  to  examine  this  point  with  some  care,  and  to 
do  this  we  must  define  a  little  more  accurately  the  mode  of 
measuring  the  magnetic  quantities  concerned.  If  a  wire 
covered  with  silk  or  cotton  is  wound  any  number  of  times 
round  a  bobbin  of  any  kind,  and  if  a  current  of  a  certain 
strength,  measured  in  amperes,  is  allowed  to  flow  through 
that  wire,  the  product  of  the  number  of  turns  of  the  wire 
and  the  strength  of  the  current  measured  in  amperes  which 
is  flowing  round  the  bobbin  wire  is  an  important  quantity, 
which  is  called  the  ampere-turns  of  that  bobbin.  The  total 
number  of  ampere-turns  on  the  bobbin,  divided  by  the  length 
of  the  bobbin,  gives  us  the  ampere-turns  per  unit  of  length 
of  the  coil.  We  have  already  seen  that  in  the  interior  of 
such  a  bobbin  of  insulated  wire  when  traversed  by  an  electric 
current,  there  is  a  magnetic  flux — or  magnetic  induction, 
as  it  is  called — the  direction  of  this  field  being  in  the  direction 
of  the  axis  of  the  bobbin.  Before  we  can  show  how  this, 
induction  is  measured  it  will  be  necessary  to  direct  attention 
to  the  fundamental  discovery  in  connection  with  this  subject. 

Imagine  that  in  the  interior  of  the  bobbin  employed  a 
moment  or  two  ago  in  our  experiments  another  bobbin  of  wire  is 
inserted,  so  as  to  occupy  a  portion  of  the  space  in  the  interior. 
Let  this  second  bobbin  be  called,  for  the  sake  of  distinction, 


19$    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

the  secondary  conducting  circuit,  whilst  the  first  is  called  the 
primary  circuit.  Faraday  discovered  that,  as  long  as  the 
electric  current  flowing  in  the  primary  circuit  remains  con- 
stant or  unaltered,  no  effect  will  be  produced  by  it  upon  the 
secondary  bobbin.  The  aperture,  or  central  hollow,  of  this 
secondary  bobbin  remains  traversed  by  part  or  all  of  the  mag- 
netic induction,  or,  as  we  have  called  it,  the  magnetic  flux 
of  the  first  bobbin,  but  otherwise  it  is  not  affected.  If  the 
electric  current  traversing  the  primary  bobbin  is  either  stopped 
or  reversed  in  direction,  or  is  altered  in  strength,  the  immediate 
result  is  to  produce  a  current  of  electricity  circulating  in  the 
secondary  bobbin ;  which  current  is,  however,  only  a  very  brief 
or  transitory  current,  and  does  not  continue  for  any  length  of 
time.  It  is,  as  it  were,  a  sort  of  wave  of  electricity  passing 
through  the  secondary  circuit.  By  appropriate  instrumental 
means  we  can  measure  the  quantity  of  electricity  which  is  in 
this  way  set  in  motion.  We  can  also  measure,  in  the  unit 
called  an  ohm,  as  explained  in  the  First  Lecture,  the  electrical 
resistance  of  this  secondary  circuit.  Faraday  showed  that  the 
value  of  the  magnetic  induction  which  is  traversing  the 
secondary  bobbin  could  be  exactly  measured  by  the  product 
of  the  resistance  of  this  secondary  circuit  and  the  whole  of 
the  quantity  of  electricity  set  flowing  in  it  when  the  primary 
current  was  suddenly  arrested.  Hence  the  use  of  a  secondary 
circuit  of  this  kind  affords  us  the  means  of  exploring  the 
magnetic  condition  of  the  space  in  the  interior  of  such  a 
primary  bobbin. 

In  Fig.  68  is  shown  a  pair  of  spiral  conducting  circuits, 
P  and  S,  which  consist  of  spirals  of  wire  wound  through 
suitable  holes  in  a  card.  If  a  current  of  electricity  is  passed 
through  the  circuit  P,  and  if  iron  filings  are  sprinkled  on  the 
card,  we  find  that  the  lines  of  magnetic  induction,  due  to  the 
primary  circuit,  are  marked  out.  Some  of  these  lines  will  be 
seen  to  pass  through,  or  be  linked  with,  the  secondary  circuit  S. 


ELECTRIC  DISTRIBUTION. 


199 


It  may  be  well  to  state  at  this  point  that  the  quantity  of 
electricity  which  flows  past  any  point  on  a  conducting  electric 
circuit  in  one  second,  when  it  is  being  traversed  by  a  current 
of  one  ampere,  is  called  one  coulomb.  We  are  able  to  measure 
quantity  of  electricity  by  means  of  an  instrument  called  a 
ballistic  galvanometer,  and,  if  we  are  provided  with  such  an 
appliance,  it  is  a  comparatively  simple  matter  to  determine 
the  whole  quantity  of  electricity  which  flows  through  a  circuit 
when  any  change  takes  place  in  the  magnetic  current  or 
induction  linked  with  it. 


+ 


T 


FIG.  6?. — Curves  delineated  by  Iron  Filings  on  a  Card,  showing  the  lines 
of  Magnetism  of  a  Spiral  Primary  Circuit,  P,  passing  through  and  linked 
with  a  Secondary  Circuit,  S. 

Returning,  then,  to  the  case  of  our  primary  and  secondary 
bobbin :  let  us,  for  the  sake  of  simplicity,  assume  that  the 
secondary  bobbin  is  wound  closely  on  the  outside  of  the  primary 
bobbin,  and  that  the  primary  bobbin  has  a  certain  given  number 
of  turns,  and  is  traversed  by  a  current  of  a  certain  strength 
measured  in  amperes.  When  a  primary  current  of,  say,  ten 
amperes  is  passing  through  the  primary  bobbin,  we  have  a 
magnetic  flux  of  a  certain  strength,  or  a  magnetic  induction 
taking  place  in  the  direction  of  the  axis  ctf  chat  primary 
bobbin,  part  or  all  of  which  magnetic  induction  or  flux 
perforates  or  passes  through  the  secondary  circuit.  If  the 
secondary  circuit  is  connected  to  a  ballistic  galvanometer, 


£00    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

and  the  primary  current  is  suddenly  stopped,  the  galvanometer 
will  give  an  indication  that  a  brief  wave  of  electricity  has 
passed  through  the  secondary  circuit,  and  this  is  called  a  secon- 
dary current.  The  product  of  the  quantity  of  electricity  in  this 
secondary  current  and  the  resistance  of  the  secondary  circuit 
is,  under  these  circumstances,  a  measure  of  the  amount  of  the 
total  magnetic  induction,  or  the  total  magnetic  flux  due  to 
the  primary  current  in  the  primary  bobbin  which  is  flowing 
through  or  linked  with  the  secondary  circuit.  In  the  above 
case  the  magnetic  circuit  consists  of  air — that  is  to  say,  the 
magnetic  flux  takes  place  in  a  circuit  which  is  merely  the  air 
space  in  and  around  the  primary  bobbin. 

Supposing  that  we  insert  in  the  interior  of  the  primary 
bobbin  a  thick  iron  rod  wholly  filling  up  the  interior  space, 
and  either  straight  or  bent  round  so  as  to  form  a  closed  ring. 
Experiment  shows  that  under  these  circumstances  the  state 
of  affairs  is  greatly  altered.  On  passing  through  the  primary 
bobbin  the  same  current  of,  say,  ten  amperes,  and  then 
stopping  that  current  suddenly,  we  should  find  that  the  electric 
impulse  produced  in  the  secondary  circuit  is  immensely  in- 
creased, and  that  the  quantity  of  electricity  circulated  in  the 
secondary  circuit  would  then  be  a  thousand  or  more  times 
greater — multiplied  probably  50  times  if  the  iron  bar  were 
a  short  straight  rod,  and  multiplied  2,000  times  if  the  iron 
was  bent  round  in  the  form  of  a  ring.  This  fact  shows  us 
that  a  given  number  of  ampere-turns  in  the  primary  bobbin 
produces  an  electric  impulse,  or  electromotive  force,  in  the 
secondary  circuit  which  is  dependent  upon  the  nature  of  the 
material  filling  the  space  in  the  interior  of,  and  outside  of, 
the  primary  bobbin. 

These  facts  can  all  be  summed  up  in  the  following  state- 
ments: If  there  be  two  circuits  of  insulated  wire,  which  are 
called  respectively  the  primary  and  secondary  circuits,  and 


ELECTRIC  DISTRIBUTION.  201 

which  are  placed  near  to  one  another  or  wound  over  one 
another,  then  the  flow  of  a  continuous  current  of  electricity 
through  the  primary  circuit  produces  no  effect  whatever  upon 
the  secondary  circuit,  so  long  as  that  primary  current  remains 
unaltered  in  strength.  If  the  primary  current  is  changed  in 
strength,  either  being  increased  or  diminished,  reversed  or 
reduced  to  nothing,  this  change  in  the  number  of  ampere  - 
turns  in  the  primary  bobbin  gives  rise  to  an  electromotive 
force,  producing  an  electric  current  in  the  secondary  circuit, 
and  this  electromotive  force  depends  essentially  upon  the 
rate  of  change  of  the  current  strength,  or  the  rapidity  of 
the  change  which  is  made  in  the  strength  of  the  primary 
current,  and,  therefore,  in  the  rate  of  change  of  strength 
of  the  magnetic  flux  or  magnetic  induction  which  is  pro- 
duced by  the  primary  current,  and  also  passes  through  the 
secondary  circuit. 

Following  a  conception  of  Faraday's,  it  is  usual  to  speak 
of  the  direction  of  the  magnetic  flux  which  surrounds  the 
electric  current  as  the  direction  of  the  magnetic  lines  of  force 
due  to  the  primary  current,  and  these  lines  of  force  can,  as  we 
have  already  explained,  have  their  directions  made  manifest 
by  the  employment  of  steel  or  iron  filings.  The  peculiar 
property  which  iron  possesses,  when  used  as  a  magnetic 
circuit,  is  that  it  permits  the  production  through  it  of  a  given 
magnetic  flux  by  the  employment  of  a  far  less  number 
of  ampere- turns  per  unit  of  length  than  does  any  non- 
magnetic material. 

We  may  examine  the  behaviour  of  iron  in  this  respect  most 
easily  by  returning  to  the  fundamental  experiment  by  which 
it  was  discovered.  Let  us  take  a  wooden  ring  having  a 
circular  section,  and  wind  over  it  insulated  copper  wire  to 
form  a  primary  circuit,  and  over  that  again  another  insulated 
secondary  circuit.  Through  the  primary  circuit  let  an  electric 


202    ELEGTEIG  LAMPS  AND  ELECTRIC  LIGHTING. 

current  be  passed,  producing,  therefore,  a  certain  magnetising 
force,  measured  by  the  number  of  ampere-turns  per  unit  of 
length  of  that  bobbin.  From  what  has  been  above  explained 
it  will  be  seen  that  this  primary  current  gives  rise  to  a 
magnetic  induction  or  magnetic  flux  which  passes  round 
the  ring,  and  some  or  all  of  which  traverses  the  secondary 
circuit.  If  the  primary  current  is  stopped,  the  arrest  of 
the  magnetic  flux  flowing  through  the  secondary  circuit 
creates  in  it  an  electromotive  force  which  causes  a  brief 
flow  of  electricity  to  take  place  through  that  circuit,  if  it  is 
complete,  and  that  electromotive  force  at  any  instant  is 


FIG.  69. — Iron  Ring  I,  having  wound  on  it  two  Circuits, — a  Primary,  P, 
connected  to  a  battery,  B,  through  a  switch  or  key,  K,  and  a  Secondary 
Circuit,  S,  connected  to  a  galvanometer,  G.  When  the  Primary  Current 
is  etopped  or  started  it  creates  an  Induced  Current  in  S. 

measured  by  the  rate  at  which  the  magnetic  flux  or  magnetic  in- 
duction passing  through  the  secondary  circuit  is  being  changed. 
Supposing,  however,  that  these  circuits,  instead  of  being  wound 
on  a  wooden  ring,  are  wound  on  an  iron  ring  of  exactly  the 
same  size  (see  Fig.  69),  and  that  we  pass  through  the  primary 
circuit  a  current  of  the  same  magnitude  as  before.  The  arrest 
or  stoppage  of  this  current  would  now  be  found  to  produce  a 
vastly  greater  electromotive  force  in  the  secondary  circuit,  and 
the  explanation  of  this  fact  is  that  the  primary  ampere-turns 
are  now  able  to  produce  a  much  greater  magnetic  flux 
through  the  secondary  circuit,  and  that,  therefore,  the  arrest 
of  the  primary  current  creates  a  greater  total  change  in 
the  magnetic  flux  flowing  through  the  secondary  circuit. 
The  nature  of  the  material  filliog  the  space,  either  partly  or 


ELECT lUG  DISTBIBUTION.  203 

wholly,  in  and  around  the  primary  and  secondary  circuits, 
has  a  very  important  influence  on  the  inductive  effect  of  the 
primary  circuit  upon  the  secondary. 

It  is  not  unusual  to  state  that  the  primary  circuit 
exerts  a  magneto-motive  force  on  the  magnetic  circuit.  In 
the  above  simple  instance  of  the  iron  ring,  we  see  that 
we  have  two  electric  circuits  and  one  magnetic  circuit, 
which  are  linked  together  like  the  links  of  a  chain,  and 
that  between  the  three  circuits  there  is  a  fixed  relation  of 
action  which  is  as  follows :  The  electromotive  force  acting 
in  the  primary  circuit  produces  in  the  primary  circuit  an 
electric  current.  This  electric  current,  flowing  a  certain 
number  of  times  round  the  primary  circuit,  produces  a 
magneto-motive  force  of  a  given  magnitude,  which  creates  a 
magnetic  flux,  or,  as  it  is  called,  a  magnetic  induction  in 
the  magnetic  circuit.  The  magnetic  circuit,  in  turn,  exercises 
an  influence  upon  the  secondary  circuit,  but  only  when  the 
magnetic  induction  or  magnetic  flux  is  being  changed.  In 
the  secondary  circuit  an  electromotive  force  is  set  up  when- 
ever the  magnetic  flux  is  being  altered  in  magnitude,  and 
the  electromotive  force  acting  to  produce  an  electric  current 
in  the  secondary  circuit  is  measured  by  the  rate  at  which  the 
magnetic  flux  or  magnetic  induction  throughout  is  being 
changed. 

We  have,  therefore,  the  following  operations  related  to  one 
another  in  these  three  circuits,  the  primary  circuit,  the 
secondary  circuit,  and  the  magnetic  circuit.  The  primary 
electromotive  force  acting  in  the  primary  circuit  produces 
a  primary  current ;  the  primary  current  acting  on  the 
magnetic  circuit  produces  a  magneto -motive  force;  the 
magneto-motive  force  produces  a  magnetic  flnx  or  mag- 
netic induction  in  the  magnetic  circuit ;  the  magnetic  in- 
duction, when  changing  in  amount,  produces  a  secondary 


204     ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

electromotive  force  in  the  secondary  circuit,  and  therefore 
a  secondary  current.  In  order  to  distinguish  that  quality 
of  iron  and  air  in  virtue  of  which  a  given  magneto  motive 
force  produces  a  much  greater  magnetic  current  or  magnetic 
induction  in  the  iron  than  it  does  in  the  air,  we  say  that 
the  iron  possesses  a  less  magnetic  reluctivity  than  the  air. 
The  magnetic  reluctivity  of  iron  may  be  as  much  as  from  one 
to  two  thousand  times  less  than  that  of  air. 

Supposing,  in  the  next  place,  we  make  a  cut  across  each 
side  of  our  iron  ring  (as  seen  in  Fig.  70),  separating  the 
iron  ring  into  two  portions  by  a  narrow  air-gap  across  each 


FIG.  70. — Iron  ring  I,  as  in  Fig.  69,  but  having  a  pair  of  air-gaps  made  by 
cutting  the  ring  in  two  at  the  points  A  A. 

side.  It  would  then  be  found  that  a  given  electric  current 
flowing  in  the  primary  circuit  would,  when  arrested,  produce  a 
much  smaller  secondary  electromotive  force  in  the  secondary 
circuit  than  it  does  when  the  iron  ring  is  uncut.  The  reason 
for  this  is  that  the  air-gap  on  both  sides  has  increased  the 
magnetic  resistance  of  the  magnetic  circuit ;  and,  accord- 
ingly although  the  magnetic  flux  or  induction  is  able 
to  traverse  the  air-gaps,  the  magneto-motive  force  of  the 
primary  circuit  is  now  not  able  to  produce  so  much  magnetic 
flux  or  magnetic  induction  in  the  magnetic  circuit ;  and 
hence,  when  the  primary  current  is  arrested,  the  change  in 
the  magnetic  induction  or  magnetic  flux  flowing  through 
the  secondary  circuit  is  not  so  great  as  before. 


1  OFTHB       'TK^V 

UNIVERSITY  ] 


X 


ELECTRIC  DISTRIBUTION. 


205 


The  two  cases  we  have  just  examined — namely,  the  iron 
ring  complete,  with  a  primary  and  a  secondary  circuit  wound 
over  it,  and  an  iron  ring  with  two  air-gaps  in  it — that  is  to 
say,  two  half- iron  rings,  on  one  of  which  is  wound  a  primary 
circuit,  and  on  the  other  a  secondary  circuit— constitute  two 
typical  models  which  will  enable  us  to  explain  the  action 
of  appliances  which  are  called  dynamo  electric  machines  and 
transformers. 


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FIG.  71.— Ii'on  Ring  magnetised  in  lines  round  the  ring,  but  producing  no 
external  magnetic  field  or  influence. 

Attention  should,  at  this  point,  be  directed  to  four 
diagrams,  which  are  reproductions  from  photographs  of 
experiments  easily  made. 

In  Fig.  71  is  shown  an  iron  ring,  circular  in  form.  Let  a 
spiral  of  insulated  wire  be  wound  on  this  ring  and  an  electric 
current  be  passed  through  it.  This  current  will  magnetise 
the  ring  along  lines  shown  as  dotted  lines.  If  iron  filings 
are  sprinkled  over  a  card  held  over  the  ring  they  will  not 


206    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

arrange  themselves  in  any  particular  form  in  the  space 
outside  the  ring,  thus  showing  that  the  magnetism  is  wholly 
confined  to  the  ring.  Next,  let  a  narrow  cut  be  made  in  the 
ring  (see  Fig.  72).  At  once  we  find,  by  applying  the  iron 
filings  test,  that  there  is  now  a  development  of  magnetism 
in  the  space  outside  the  ring,  and  that  there  are  lines  of 
magnetic  induction  or  flux  passing  across  from  one  side 
of  the  air-gap  to  the  other,  producing  a  powerful  magnetic 


FIG.  72. — Magnetised  Iron  King  with  cut  or  air-gap.  The  ring  is 
magnetised  along  the  dotted  lines.  The  external  magnetic  field  or  induc- 
tion is  delineated  by  iron  filings. 

field  in  the  gap  space.  By  making  wider  gaps,  as  shown  in 
Figs.  73  and  74,  the  varying  forms  of  the  external  fields  can 
be  detected  by  the  use  of  the  sprinkled  filings. 

We  have  already  seen  that,  in  order  to  produce  an  induced 
electromotive  force,  and  therefore  an  electric  current,  in  the 
secondary  circuit,  it  is  necessary  to  produce  a  change  in  the 
magnetic  flux  or  magnetic  induction  passing  through  that 


ELECTRIC  DISTRIBUTION. 


207 


FIG.  73. — Magnetised  Iron  Ring,  as  in  Fig.  72,  but  with  wider  air-gap. 


/i;/V>^^>:-;t:::,}V/: 

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•'•"i/--  j  :••;•* '*>^A  ^.^-  '•  "A '-*,  '''  '   •»'''•'- 1 J    .'.   '-    .'T 
74,__Magnetised  Iron  Ring,  as  in  Fig.  72,  but  with  still  wider  air-gap. 


208     ELECTEIC  LAMPS  AND  ELECTEIG  LIGHTING. 

circuit.  This  change  may  be  produced  in  two  ways — either  by 
stopping  the  primary  current  or  by  reversing  its  direction. 
In  the  second  case  the  amount  of  change  in  the  induction  is 
doubled,  because  the  reversing  of  the  primary  current  amounts 
to  first  stopping  it,  and  then  starting  it  again  in  the  opposite 
direction.  Take  the  first  case,  viz.,  the  complete  iron  ring, 
and  imagine  the  direction  of  the  primary  current  continually 
being  changed  or  alternated.  This  would  produce  an  alter- 
nating magnetic  flux  round  the  ring,  the  direction  of  the 
magnetisation  changing  its  direction  with  every  change  in  the 
direction  of  the  primary  current.  This  changing  magnetism 
passing  through  the  secondary  circuit  would  set  up  in  the 
secondary  circuit  an  alternating  secondary  electromotive  force, 
and  therefore,  if  the  secondary  circuit  is  completed,  an  alter- 
nating secondary  current.  It  will  be  at  once  evident,  however, 
that  there  are  two  ways  in  which  we  may  set  up  this  changing 
magnetism  in  that  half  of  the  ring  on  which  the  secondary 
circuit  is  wound.  We  may  either  employ  the  undivided  ring, 
and  keep  continually  changing  the  direction  of  the  primary 
current  in  the  primary  circuit,  or  we  may  maintain  the  primary 
magnetising  current  constant  in  one  half  of  the  ring  and  turn 
round  that  half  of  the  ring  on  which  the  secondary  circuit  is 
wound,  so  as  to  keep  on  reversing  the  direction  of  the  magnetic 
current  through  the  secondary  circuit.  A  little  consideration 
of  these  two  modes  of  producing  the  changing  induction  through 
the  secondary  circuit  will  show  that  they  really  amount  to  the 
same  thing. 

In  the  light  of  the  above  remarks  it  will  be  found  very  easy 
to  understand  the  general  action  of  the  alternate  current 
transformer  and  dynamo.  It  has  been  explained  in  the  previous 
Lectures  that  the  power  conveyed  by  a  continuous  electric 
current  is  measured  by  the  product  of  the  current  strength 
measured  in  amperes  and  the  difference  of  pressure  or  poten- 
tial between  the  ends  of  the  circuit  measured  in  volts,  The 


ELECTRIC  DISTRIBUTION.  209 

same  amount  of  power,  say  one  horse-power,  can,  therefore, 
be  conveyed,  either  in  the  form  of  a  large  current  having  a 
small  fall  in  pressure  or  in  the  form  of  a  small  current  having 
a  large  fall  in  pressure.  We  are  familiar  enough  with  this 
effect  in  the  case  of  water.  A  waterfall  may  consist  of  a  very 
large  body  of  water  falling  down  a  very  moderate  height,  such 
as  the  well-known  falls  of  the  Rhine  at  Neuhausen,  in  Switzer- 
land ;  or  it  may  present  itself  in  the  form  of  a  much  less 
quantity  of  water  falling  from  a  much  greater  height,  like  the 
falls  of  the  Anio,  at  Tivoli,  in  Italy.  In  either  case  the  power 
which  the  water  would  be  capable  of  exerting,  if  utilised  by 
means  of  turbines  or  waterwheels,  would  depend,  not  on  the 
height  of  the  fall  nor  on  the  quantity  of  the  water  flowing  down, 
taken  alone,  but  on  the  product  of  the  numbers  representing 
respectively  the  height  and  the  quantity.  The  height  of  the 
fall  determines  the  velocity  or  pressure  of  the  water  at  the 
base  of  the  fall.  It  is  impossible  to  get  any  work  out  of 
water  unless  we  are  able  to  let  it  fall  from  one  level  to  a 
lower  one.  Hence  it  is  that  stores  of  water  at  a  height  above 
the  level  of  the  sea  represent  available  sources  of  energy. 

The  power  which  originally  lifted  the  water  up  before  it 
can  fall  must  have  been  the  evaporative  power  of  the  sun's 
heat.  We  commonly  speak  of  using  water-power',  but,  as 
a  matter  of  fact,  we  are  really  employing  in  these  cases 
sun-poiver.  The  falls  of  Niagara  represent  the  accumu- 
lated water  drainage  of  half  a  continent  falling  over  a 
precipice  of  150  feet  in  height  on  its  way  to  the  sea.  This 
water,  however,  was  originally  evaporated  from  lakes,  rivers, 
or  the  sea,  before  it  could  be  precipitated  as  rain  over 
Canada  and  ^orth  America.  The  power  which  lifted  these 
water  molecules  from  the  surface  of  the  Atlantic  or  Pacific 
Oceans  into  a  position  in  which  we  are  able  to  make  them 
give  back  part  of  their  energy  of  position  was  the  radiation 
proceeding  from  the  sun.  The  dynamos  now  being  placed  in 


210    ELECTMIC  LAMPS  AND  ELECTRIC  LIGHTING. 

position  to  tap  off  some  of  the  available  power  of  the  Falls  of 
Niagara  will  in  reality  be  driven  by  sun-heat,  radiated  to  the 
earth  at  some  previous  time. 

These  hydraulic  facts  have  an  exact  parallel  in  electrical 
science.  A  current  of  electricity  can  only  do  work  by  falling 
down  from  one  electric  pressure  or  potential  to  a  lower  one, 
and  the  rate  at  which  it  can  do  work  is  measured,  not  by 
the  current  strength  or  by  the  fall  in  potential  alone,  but  by 
the  product  of  the  numbers  representing  these  two  quantities. 
A  current  of  electricity  of  10  amperes  falling  in  pressure  by 
1,000  volts,  represents  the  same  power  of  doing  work  as  a 
current  of  1,000  amperes  falling  10  volts  in  pressure.  There 
is,  however,  a  considerable  difference  between  the  two  agents. 
The  small  current  of  high  pressure  is  very  different  in  many 
respects  from  the  large  current  of  low  pressure.  In  the  first 
place,  the  high-pressure  current  is  more  dangerous  to  handle 
or  deal  with,  and  requires  much  better  insulation  than  the 
low-pressure  current.  On  the  other  hand,  when  a  current  of 
electricity  flows  through  a  conductor  a  part  at  least  of  its 
energy  is  frittered  away  into  heat  by  reason  of  the  necessary 
resistance  of  the  conductor.  The  amount  so  dissipated  is,  by 
Joule's  law,  measured  by  the  product  of  the  square  of  the 
current  strength  and  the  resistance  of  the  conductor  measured 
in  ohms.  Hence,  if  flowing  in  conductors  of  the  same  size  in 
cross-section  and  length,  a  current  of  1,000  amperes  would 
generate  10,000  times  more  heat  per  second  than  a  current  of 
10  amperes.  Suppose,  then,  that  we  wish  to  convey  a  power 
of  10,000  watts  to  a  distance,  we  can  do  it  either  by  using  a 
current  of  10  amperes  flowing  out  and  back  along  electric 
mains  between  which  1,000  volts  pressure  is  maintained,  or  by 
using  a  current  of  100  amperes  flowing  in  mains  between  which 
100  volts  pressure  difference  is  preserved.  If,  however,  we 
wish  to  dissipate  or  waste  only  the  same  fraction  of  our  10,000 
watts  power,  say  10  per  cent.,  in  conveying  it,  we  shall  have, 


ELECTRIC  DISTRIBUTION.  211 

in  the  case  of  low  pressure  current,  to  employ  electric  mains 
of  100  times  less  resistance  in  the  second  case  than  in  the 
first.  In  other  words,  we  must  provide  a  conducting  channel 
for  the  large  current  of  100  amperes  which  shall  be  100  times 
the  cross-section,  and  therefore  100  times  the  conductivity,  of 
that  which  it  would  be  necessary  to  lay  down  for  conveying 
the  small  current  of  10  amperes,  with  the  same  proportionate 
loss  of  energy.  It  will,  therefore,  be  evident  that,  as  far  as 
mere  cost  of  copper  is  concerned,  it  is  cheaper  to  convey 
electric  energy  in  the  form  of  small  electric  currents  at  high 
pressure  than  as  a  large  electric  current  at  lower  pressure. 
On  the  other  hand,  the  high  pressure  conductors  cost  more 
to  insulate,  so  the  advantage  is  not  simply  proportionate  to 
the  economy  in  copper. 

The  point  to  which  I  wish  now  to  direct  your  attention  is 
that  we  have  in  the  transformer  an  appliance  which  enables 
us  to  convert  small  currents  of  high  pressure  into  large 
currents  of  low  pressure.  In  so  doing  we  do  not  create  any 
energy — in  fact,  we  waste  some  of  it ;  but  the  transformer  is 
a  device  which  enables  us  to  change  the  form  of  electrical 
power  just  as  a  pulley  or  other  simple  machine  enables  us  to 
change  the  form  of  mechanical  power.  By  the  use  of  a  pulley 
and  tackle  a  man  exerting  a  small  force  through  a  great 
distance  can  raise  a  very  heavy  weight  through  a  smaller 
distance. 

The  alternate-current  transformer  is  an  appliance  which 
operates  on  electric  power  just  as  a  lever  or  pulley  does  on 
mechanical  power.  It  consists  of  an  iron  ring  or  circuit  of 
some  form  or  other  which  is  wound  over  with  two  wires,  a 
thick  wire  and  a  thin  one.  These  circuits  are  carefully 
insulated  one  from  the  other.  The  thin  wire  generally  makes 
some  ten  or  twenty  times  more  turns  round  the  iron  ring 
than  the  thick  one.  If  a  small  alternating  current  of  elec- 

p2 


212    ELECTRIC  LAMPS  AND  ELECTRIC 

tricity  from  a  high  pressure  source  is  passed  through  the  thin 
wire,  it  creates  an  alternating  magnetism  in  the  ring  of  iron, 
magnetising  it  first  one  way  and  then  the  other  in  a  direc- 
tion round  the  axis  of  the  ring.  This  changing  magnetism 
creates,  as  before  explained,  another  alternating  electric  current 
in  the  thick-wire  circuit,  called  a  secondary  current,  and  this 
current  can  be  made  to  have  a  lower  pressure  and  greater 
strength  than  the  primary  current  by  suitably  proportioning 
the  number  of  windings  of  the  two  circuits.  Hence  we  can 
transform  a  small  current  produced  by  a  pressure  of  2,000 


FIG.  75. — One  form  of  an  Alternate- Current  Transformer  in  process  of 
manufacture. 


volts  into  a  current  nearly  20  times  as  strong,  but  having  a 
pressure  only  of  100  volbs.  In  this  transformation  there  is 
a  certain  loss  of  power,  or  dissipation  of  energy,  but  in  good 
transformers  this  will  not  exceed  from  1  to  4  per  cent,  of  the 
amount  of  power,  transformed.  We  can,  then,  by  means  of 


ELECTRIC  DISTRIBUTION. 


213 


the  transformer,  carry  electric  power  at  high  pressure  and 
transform  it  down  to  a  low  pressure  at  the  place  where  it  is 
to  be  used.  In  transformer  systems  of  electric  supply  this 


is  the  method  adopted,  as  opposed  to  the   other  or  direct- 
current  supply,  in  which  continuous  currents  flowing  uniformly 


214     ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

in  one  direction  are  used,  and  which  distribute  current  at 
a  lower  pressure  for  use  without  transformation. 

The  practical  construction  of  the  transformer  is  carried  out 
as  follows  : — A  number  of  thin  iron  bands,  strips,  or  plates  are 
so  arranged  as  to  form  an  iron  ring,  or  its  equivalent.  This 
ring  is  then  wound  over  with  two  circuits  of  insulated  wire, 
one  a  fine  wire,  called  the  primary  circuit,  and  one  a  thick 
wire,  called  the  secondary.  In  Fig.  75  is  shown  such  a  trans- 
former in  process  of  manufacture. 

The  transformer,  when  completed,  is  generally  enclosed  in 
an  iron  case,  and  is  provided  with  proper  terminals  for  the 
two  circuits  (see  Fig.  76).  Transformers  made  as  above  are 
generally  constructed  for  various  transforming  powers,  such 
as  1  horse-power  (H.P.),  10  H.P.,  &c.  This  means  that  the 
transformer,  say  of  10  H.P.,  can  transform  the  energy  of  a 
current  of  about  4  amperes  at  a  pressure  of  2,000  volts  into 
energy  in  the  form  of  a  current  of  about  74  amperes  at  a 
pressure  of  100  volts.  The  sizes  of  transformers  are  often 
given  in  kilowatts,  and  then  we  have  to  remember  that  one 
kilowatt  is  equal  to  about  1£  H.P.  Hence  a  30-kilowatt  trans- 
former is  a  40-H.p.  transformer.  It  will,  therefore,  be  seen 
that  a  transformer  is  only  a  development  of  the  simple  iron 
ring  wound  over  with  two  circuits,  as  described  at  page  180, 
and  illustrated  in  Fig.  69.  The  object  of  building  up  the 
iron  core,  or  ring  of  iron  strips  or  plates,  is  to  check,  as  far 
as  possible,  the  production  of  electric  currents  in  the  mass  of 
the  iron  core,  which,  if  unprevented,  would  be  a  source  of 
waste  of  power. 

We  are  also  able  to  present  a  similar  simple  explanation  of 
the  mode  of  action  of  the  dynamo  machine.  Kef  erring  again 
to  Fig.  70,  we  see  that  we  can  reverse  or  change  the  direc- 
tion of  the  magnetic  induction  or  current  flowing  through 


ELECTRIC  DISTRIBUTION. 


215 


the  secondary  circuit  by  making  two  cuts  or  air-gaps  in  tha 
iron  ring,  and  then  rotating  that  half  of  the  ring  on  which 
the  secondary  circuit  is  bound.  This  can,  perhaps,  be  better 
understood  by  reference  to  another  diagram  (see  Fig.  77).  In 
this  figure  the  shaded  horse-shoe  shaped  part  marked  M  is 
supposed  to  represent  an  iron  core,  which  is  magnetised  by  an 
electric  current  passed  through  a  wire  wound  round  that  core, 
but  which  wire,  for  the  sake  of  simplicity,  is  not  shown  on  the 
diagram.  This  magnetic  circuit  is  completed  by  placing 
another  iron  mass,  represented  in  the  figure  by  the  shaded 
rectangle  in  between  the  poles  or  ends  of  the  horse-shoe 
magnet.  We  thus  obtain  an  iron  circuit  having  two  cuts 
or  air-gaps  in  it,  corresponding  with  the  split  ring  shown  in 
Fig.  70. 


FIG.  77. — Skeleton  Diagram,  showing  the  principle  of  the  Dynamo  Machine. 

Suppose,  now,  a  magnetic  induction  or  magnetic  flux 
to  exist  in  one  direction  round  this  air-iron  circuit,  and 
to  be  produced  by  a  primary  current  flowing  in  a  primary 
circuit  of  insulated  copper  wire  which  is  wound  on  the  horse- 
shoe shaped  iron  core.  This  magnetic  induction  or  flux 
will  pass  across  the  air-gaps,  producing  there  a  powerful  mag- 
netic field.  Next,  let  us  suppose  that  the  iron  block,  or,  as  it 
is  called,  the  armature  core,  is  wound  over  with  a  circuit  of 
wire  indicated  by  the  rectangle  A  A  in  Fig.  77.  Let  this  core 
and  armature  wire  be  capable  of  rotating  round  an  axis,  and 
suppose  that,  by  a  couple  of  contact  rings  B  B,  we  can  always 
keep  the  ends  of  the  armature  circuit  A  A  in  conducting 


216    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

connection  with  an  external  circuit  C.  If  the  rectangular 
core  and  its  wire  winding  is  made  to  revolve  round  its  axis,  it 
is  easy  to  see  that  the  direction  of  the  magnetic  flux  or 
induction  through  this  secondary  or  armature  circuit  will  be 
continually  reversed,  and  hence  that  an  induced  secondary 
current  will  be  created  in  it,  which  will  also  be  continually 
reversed — that  is,  it  will  be  an  alternating  current. 

A  dynamo,  therefore,  consists  essentially  of  an  iron  core, 
or  field  magnet,  as  it  is  termed,  which  is  wound  over  with  a 
primary  circuit  or  field  wire  circuit,  and  this  circuit,  when 
traversed  by  an  electric  current,  creates  a  powerful  magnetic 
field  in  the  space  between  the  poles  or  ends  of  the  field 
magnet.  In  the  interpolar  space  is  placed  an  iron  cylinder 
or  ring,  called  the  armature  core,  and  this  serves  to  complete 
the  magnetic  circuit.  On  this  armature  core  is  wound  another 
coil  or  coils  of  wire  called  the  armature  windings,  which 
correspond  to  the  secondary  circuit  of  the  transformer.  The 
armature  core  and  its  windings  are  rotated  by  being  fixed 
on  a  spindle  to  which  is  usually  attached  a  pulley.  In 
Fig.  78  is  shown  a  representation  of  an  Edison-Hopkinson 
dynamo,  in  which  the  field  magnets  and  armature  are  easily 
distinguished.  In  one  large  class  of  dynamos,  called  alternate- 
current  machines,  or  alternators,  the  secondary  current  which 
is  sent  out  from  the  secondary  or  armature  circuit  is,  as 
observed  above,  an  alternating  current. 

For  many  purposes,  however,  it  is  essential  to  produce  a 
continuous  current,  or  a  current  always  in  one  direction. 
To  obtain  this,  an  additional  organ  has  to  be  furnished 
to  the  dynamo.  This  is  called  a  commutator,  the  function 
of  which  is  to  convert  the  alternating  current  produced 
in  the  armature  coils  into  a  continuous  current  in  the 
external  circuit.  A  little  attentive  consideration  will,  how- 
ever, show  that  a  dynamo  and  transformer  are  essentially 


ELECTRIC  DISTRIBUTION. 


217 


the  same  appliances  in  nature,  only  that  in  the  latter  the 
necessary  reversal  of  the  flow  of  magnetism  through  the 
secondary  circuit  is  obtained  by  alternating  or  reversing  the 


FIG.  78. — Edison-Hopkinson  Dynamo. 

direction  of  the  primary  current,  whereas  in  the  case  of  the 
dynamo  the  necessary  reversal  is  obtained  by  rotating  or 
turning  round  that  part  of  the  iron  magnetic  circuit  on  which 


218     ELECTEIC  LAMPS  AND  ELECTRIC  LIGHTING. 

the  secondary  circuit  is  wound.  This  rotation  of  the  arma- 
ture of  the  dynamo  may,  in  practice,  be  effected  either  by 
putting  a  pulley  on  the  shaft  and  driving  it  by  a  belt  or  rope, 
or  by  coupling  the  dynamo  shaft  directly  to  the  shaft  of  a 
high-speed  steam  engine. 

One  of  the  methods  of  direct  driving,  as  this  is  called,  most 
widely  adopted  in  England  is  to  employ  a  high-speed  Willans 
engine,  the  shaft  of  which  is  continuous  with  that  of  the 


FIG.  79.— View  of  an  Alternator  direct  driven  by  being  coupled  to  a 
Willans  Engine. 

dynamo.  In  Fig.  79  is  shown  an  illustration  of  such  a 
combination.  It  is  obvious  that  this  arrangement  is  exceed- 
ingly economical  of  space,  and  hence  has  been  widely  adopted 
in  the  arrangement  of  electric  generating  stations,  in  which 
space  is  of  importance. 

It  is  impossible,   as   observed   at   the   beginning  of  this 
Lecture,  to  do  more  than  give  here  the  briefest  outline  of  the 


ELECTRIC  DISTRIBUTION.  219 

different  methods  employed  for  distributing  current  for  illu- 
minating purposes ;  but  the  non-technical  reader  will  probably 
be  able  to  grasp  most  easily  the  nature  of  the  various 
systems  employed  by  my  selecting  one  or  two  typical 
instances,  and  describing  these  somewhat  in  detail.  At  the 
present  moment  there  are  two  principal  systems  of  electric 
lighting,  both  of  which  have  their  advantages  :  these  are 
called  respectively  the  alternating  current  supply  system 
and  the  continuous  current  supply  system.  In  the  first  of 
these  a  generating  station  is  established,  which  is  provided 
with  dynamo  -  electric  machines  called  alternators,  which 
are  capable  of  producing  an  electric  current  alternating 
in  direction  a  large  number  of  times  in  a  second.  In 
the  case  of  alternating  currents  the  current  flows  in  one 
direction  in  the  circuit  for  a  short  fraction  of  a  second,  is 
then  reversed,  and  then  flows  in  the  opposite  direction  for  a 
short  fraction  of  a  second,  and  repeats  the  same  cycle  of 
operations  continuously.  The  number  of  times  per  second 
which  the  current  cycle  is  repeated— namely,  a  flow  in  one 
direction  succeeded  by  a  flow  in  the  opposite  direction, 
like  the  ebb  and  flow  of  the  tide— is  called  the  frequency 
of  the  electric  current,  and  the  frequencies  most  usually 
employed  are  either  40,  83,  100  or  about  125.  In  England 
the  practice  generally  is  to  use  a  frequency  of  80  or  100, 
on  the  continent  of  Europe  the  lower  frequency  of  40,  and 
in  America  the  higher  of  120.  This  alternating  current  is 
generated  by  machines  called  alternators  at  a  high  pressure, 
which  is  generally  either  1,000,  2,000  or  2,400  volts,  and 
in  some  cases  5,000  or  10,000.  This  high-pressure  alter- 
nating current  is  then  led  out  from  the  station  by  highly 
insulated  conductors,  which  are  called  the  primary  mains, 
and  these  primary  mains  are  generally  made  in  what  is 
called  the  concentric  form.  A  stranded  copper  cable  is  covered 
over  with  insulating  material,  and  then  a  circular  strand  of 
wire  is  plaited  over  the  insulating  material,  and  this  again 


220    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

is  protected  by  a  further  layer  of  insulation,  and  finally  by  a 
covering  of  steel  wires,  called  the  armour.  In  Fig.  80  is 
shown  a  section  of  this  steel- armoured  concentric  cable. 

This  armoured  cable  is  laid  down  in  the  streets  under 
the  pavements,  and  is  then  usually  conducted  to  certain 
places  which  are  called  transformer  sub-centres.  These  are 
small  rooms,  either  excavated  out  underground  or  built 
abovo  ground,  and  to  them  the  high-pressure  primary 


FIG.  80.—  Cross-section  of  a  Concentric  Lead-covered  Steel -Armoured 
Cable,  for  High-Pressure  Transmission.  The  central  white  dots  represent 
the  inner  conductor,  and  the  outer  circle  of  white  dots  the  several  wires 
composing  the  outer  conductor.  The  shaded  portion  shows  the  outer 
covering  of  lead. 

current  is  brought  by  these  "primary"  cables.  In  these 
transformer  sub -centres  are  placed  a  number  of  trans- 
formers, which,  as  already  explained,  are  connected  with 
the  primary  mains,  so  that  all  their  primary  circuits  are 
in  parallel  across  the  mains.  The  secondary  circuits  of 
the  transformers  are  then  connected  to  another  set  of 
underground  mains,  which  are  called  the  secondary  dis- 
tributing mains,  and  these  distributing  mains  are  laid  down 
in  the  streets  which  are  to  be  furnished  with  light.  It  is 


ELECT&IC  DISTRIBUTION. 


221 


most  usual  to  employ  a  transformation  ratio  of  20  to  1, 
so  that  the  pressure  of  2,000  volts  is  reduced  to  100  volts 
at  the  other  side  of  the  transformer. 

In  many  cases  the  secondary  distributing  system  is  laid  down 
on  the  three- wire  system  (see  Fig.  81).  In  the  transformer 
sub-centre  are  placed  a  number  of  transformers,  T  T,  which 
have  primary  and  secondary  switches,  by  which  their  primary 
circuits  can  be  connected  to  the  primary  mains  MM,  and  their 
secondary  circuits  to  the  secondary  mains  ABC.  During  the 


—  X 

V/WV 
li 

r 

A 

S/VN/V 
If. 

T 
A 

6. 

666 

JTT"? 

P    P    P 

P    P    P 

FIG.  81.— System  of  Distribution  by  Alternating  Currents  and  Three- 
wire  Secondary  Circuits.  A  is  the  Alternating  Current  Dynamo,  M  M 
the  Primary  Mains,  T  T  the  Transformers,  and  A  B  and  C  the  Three- 
wire  Secondary  Circuits  with  Lamps  across  them. 

hours  of  small  demand  (or  "  light  load,"  as  it  is  technically 
called)  only  one  of  these  transformers,  which  is  called  the 
master  transformer,  is  kept  connected  to  the  system;  but 
during  the  hours  of  heavy  demand  (or  "  full  load  ")  two  or 
more  transformers  are  added  on,  so  as  to  share  the  load.  In 
some  cases  of  very  scattered  lighting,  instead  of  collecting 
together  the  transformers  in  sub -centres,  one  or  more  trans- 
formers are  placed  in  each  house  or  building  which  is  to 
be  supplied  with  current. 

In  such  a  transformer  system  the  sources  of  losses  of  energy 
are  as  follow  :  There  is  a  certain  constant  loss  of  energy  in 


222     ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

every  transformer  which  is  kept  connected  to  the  primary 
mains,  owing  to  the  fact  that  the  incessant  reversal  of  the 
magnetisation  of  the  iron  core  of  the  transformer  necessitates 
an  expenditure  of  energy.  This  is  called  the  "  open  circuit 
loss"  of  the  transformer,  and  in  thoroughly  efficient  trans- 
formers does  not  exceed  from  one  to  two  per  cent,  of  the 
whole  of  the  nominal  output  of  the  transformer.  Thus, 
for  instance,  if  the  transformer  is  one  which  is  capable 
of  transforming  10  horse-power  of  electric  energy  from  the 
form  in  which  it  exists  as  a  current  of  about  four  amperes 
flowing  under  a  pressure  of  2,000  volts  into  electric  energy 
which  exists  in  the  form  of  75  amperes  at  a  pressure  of 
100  volts,  such  a  transformer  can  be  made  to  dissipate  or 
waste  only  about  one- seventh  of  a  horse- power  in  the  con- 
tinual magnetisation  of  its  iron  core.  When  the  transformer 
is  doing  its  full  work,  or  is  loaded  up  to  any  extent,  in 
addition  to  this  open  circuit  loss — which  is  also  called  the 
iron  loss  in  the  transformer,  because  it  takes  place  in  the  iron 
core — there  is  an  additional  loss,  which  is  called  the  "  copper 
loss,"  and  this  is  due  to  the  heating  effect  produced  by  the 
currents  in  the  primary  and  secondary  copper  coils  of  the 
transformer.  In  well-designed  transformers  the  copper  loss 
at  full  load  is  about  equal  to  the  iron  loss  at  no  load,  and 
it  will  therefore  be  easily  seen  that  the  result  is  to  give  the 
transformer  an  "  efficiency,"  as  it  is  called,  of  about  97  per 
cent,  at  full  load — that  is  to  say,  of  the  whole  power  supplied 
to  the  transformer  in  the  form  of  high -pressure  current  97 
per  cent,  is  given  out  by  the  transformer  in  the  form  of  low- 
pressure  current.  It  is  possible  to  construct  a  transformer  so 
as  to  have  as  much  as  90  per  cent,  efficiency  at  -f^ih  of  full 
load.  The  most  important  point  to  consider,  however,  is,  not 
the  efficiency  of  the  transformer  at  full  load,  or  its  efficiency  at 
light  loads,  but  to  consider  the  so-called  all-day  efficiency  of  the 
transformer — that  is  to  say,  if  the  transformer  is  employed  for 
24  hours,  doing  such  lighting  as  may  be  demanded  of  it,  the 


ELECTRIC  DISTRIBUTION.  223 

important  quality  which  it  should  possess  is  that  of  having  a 
high  all-day  efficiency ;  in  other  words,  the  ratio  between  the 
total  number  of  units  of  electrical  energy  which  are  taken  out 
of  the  transformer  must  bear  a  large  proportion  to  the  number 
put  into  it. 

As  already  explained  in  the  Lecture  on  "  Electric  Glow 
Lamps,"  the  ordinary  demands  for  lighting  in  residential  or 
public  buildings  are  of  so  varied  a  nature  that  the  full  current 
for  the  whole  of  the  lamps  placed  in  them  is  only  required  for  a 
very  small  portion  of  the  24  hours.  For  instance,  in  an  ordinary 
private  residence,  during  the  early  part  of  the  day  there  may 
be  a  small  demand  for  lighting  in  the  lower  part  of  the  house ; 
during  the  greater  part  of  the  day  very  little ;  in  the  afternoon 
some  more  lamps  will  be  turned  on ;  in  the  evening  (in  the 
winter  between  4  and  10  or  11)  there  will  be  a  variable  but 
much  larger  demand ;  and  during  the  night  very  little.  If  on 
a  piece  of  paper  a  line  is  taken  and  divided  into  twenty-four 
parts,  and  a  perpendicular  drawn  at  each  of  those  points 
representing  by  its  altitude  the  number  of  amperes  of  current 
taken  by  the  house  at  that  moment,  a  curve  joining  the 
tops  of  all  these  lines  gives  us  the  load  diagram  of  the 
house,  and  the  area  of  this  load  diagram  gives  the  whole 
quantity  of  current  taken  by  the  house.  If,  instead  of 
erecting  perpendiculars  proportional  to  the  current,  we  erect 
perpendiculars  which  are  proportional  to  the  power  measured 
in  watts  being  taken  up  in  the  house  at  that  time,  we  get 
the  watt-Jiour  diagram,  and  the  whole  area  of  this  diagram 
measures  the  total  amount  of  energy  in  Board  of  Trade 
units  taken  up  in  the  house  during  the  twenty-four  hours. 
-The  ratio  between  this  amount  and  the  full  amount  which 
would  be  taken  if  all  the  lamps  were  kept  burning  for  the 
whole  period  is  called  the  load  factor  of  the  house,  and  the 
load  factor  of  the  house  is  on  an  average  not  more  than  10 
per  cent. 


224     ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

Accordingly,  if  a  house  or  a  series  of  houses  are  supplied 
from  one  transformer  centre,  the  actual  demand  at  any 
one  moment  for  electric  power  may  be  very  small ;  and  the 
important  point  with  regard  to  transformers  is  that  they 
should  possess  a  very  high  efficiency  at  low  loads,  in  order 
that,  on  the  whole,  the  number  of  units  of  electrical  energy 
that  are  wasted  in  the  transformers  in  magnetising  the  iron 
cores  may  not  be  a  very  large  proportion  of  the  total  energy 
which  is  supplied  to  the  houses.  Transformers  can  now  be 
easily  made  to  have  an  80  per  cent,  all-day  efficiency,  and, 
under  these  circumstances,  the  total  amount  of  energy  supplied 
to  the  customers  may  be,  and  often  is,  as  much  as  80  per  cent, 
of  that  which  is  sent  out  from  the  station.  In  inferior 
systems  of  alternating  current  supply  the  efficiency  of  distri- 
bution may  fall  very  far  short  of  80  per  cent.,  not  amounting 
to  even  more  than  50  per  cent. 

Having  placed  these  general  principles  before  our  minds, 
we  can  now  proceed  to  consider  the  main  features  of  an 
existing  alternating  current  supply  system,  which  has  been 
worked  out  with  great  care  and  forethought,  namely,  that 
adopted  for  the  supply  of  electric  energy  for  the  lighting  of 
the  city  of  Rome.  The  electric  lighting  of  Eome  is  conducted 
from  two  electric  lighting  stations,  one  of  which  has  been 
established  in  the  gas  works  at  Rome,  at  the  foot  of  the 
Palatine  Hill,  and  the  other  at  Tivoli,  eighteen  miles  from 
Rome.  The  first  station  was  put  down  in  the  year  1889  with 
the  object  of  supplying  current  for  incandescent  and  arc 
lighting  within  the  city.  This  station  occupies  the  site  of 
the  old  Circus  Maximus,  and  at  the  present  moment  it 
supplies  an  equivalent  of  nearly  40,000  10-c.p.  incandescent 
lamps.  The  station  itself  is  a  substantial  brick  building  in 
close  contiguity  to  the  gasworks.  It  contains  six  Ganz  alter- 
nating current  dynamos — four  of  600  horse-power  and  two 
of  250.  Each  alternator  consists  of  a  series  of  armature 


ELECTRIC  DISTRIBUTION. 


225 


coils  which  are  contained  on  the  inside  of  an  iron  ring  frame. 
A  central  wheel,  which  constitutes  the  flywheel  of  the  engine 


FIG.  82.— 600  horse-power  Alternator|in  the   Cerchi/  Electric  Station  at 

Rome. 


FIG.  83.— 600  horse-power  Alternator  in  (he  Cerchi   Electric  Station  at 

Rome. 


driving  the  dynamo,  carries  on  its  external  surface  a  series  of 
insulated  coils,  which  are  called  the  field-magnet  coils.     The 


226     ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

engines  are  coupled  direct  to  this  flywheel,  the  cylinders  of 
the  engines  being  one  on  either  side.  In  Figs.  82  and  83  are 
shown  views  of  these  600  H.P.  alternators  with  the  direct- 
coupled  engines.  Smaller  alternators  of  250  H.P.  are  used 
during  the  daytime,  or  at  a  time  when  the  demand  for  light- 
ing is  not  great.  These  alternators  generate  an  alternating 
current  at  a  frequency  of  40  and  a  pressure  of  2,000  volts. 


FIG.  84.— View  of  the  Hydraulic  Tower  at  Tivoli. 

They  send  their  current  into  three  primary  cables  or  feeders, 
which  are  concentric  steel- armoured  conductors,  and  these 
feeders  deliver  current  to  certain  transformer  centres  and  to 
transformers  which  are  placed  in  the  various  buildings  to  be 
lighted.  The  total  length  of  mains  laid  down  is  about  25 
kilometres,  or  16  miles.  At  the  end  of  1892  there  was  a  total 


ELECTRIC  DISTRIBUTION. 


227 


number  of  lamps  connected  to  the  system  equal  to  34,464 
10-c.p.  lamps.  In  the  transformer  centres  and  in  the  houses 
are  placed  transformers  for  reducing  the  pressure  from  2,000 
volts  to  100  volts,  two  sizes  of  transformers  being  mostly 
used,  one  capable  of  transforming  power  equal  to  10,000 


FIG.  85. — View  of  Power  Station  at  Tivoli. 

watts  and  the  other  equal  to  transforming  power  of  5,000 
watts.  From  these  transformers  secondary  cables  are  laid, 
distributing  secondary  current  at  a  pressure  of  100  volts 
to  various  buildings. 


228    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

The  capacity  of  this  station  being  fully  exhausted,  it  was 
determined  to  furnish  an  additional  supply  by  means  of  a 
second  station,  and  to  take  advantage  for  this  purpose  of  the 
large  available  water  supply  of  Tivoli.  The  village  of  Tivoli, 
which  occupies  the  site  of  the  ancient  Tibur,  stands  on  a  spur 
of  the  Sabine  Hills.  The  beauties  of  Tivoli  and  its  surrounding 
country  were  celebrated  in  undying  verse  by  Horace,  whose 
Sabine  farm  was  not  very  far  distant.  The  chief  natural 
attraction  is  the  fine  cascade  formed  by  the  Eiver  Anio,  one 
fall  of  which  is  340  feet  in  height.  Here  also  in  classic  times 
stood  Hadrian's  villa,  of  which  the  grounds  once  covered  an 
area  of  several  square  miles,  and  contained  an  unrivalled 
collection  of  works  of  art.  Upon  this  romantic  spot  the 
modern  engineer  has  laid  his  hands,  and  has  compelled  a  large 
portion  of  the  power  running  to  waste  in  these  waterfalls  to  be 
directed  to  the  purpose  of  electrically  lighting  modern  Eome. 
With  this  object,  a  hydraulic  canal  was  first  constructed, 
leading  from  one  of  the  upper  reaches  of  the  Eiver  Anio  to 
the  top  of  a  tall  tower  (see  Fig.  84).  This  tower  contains 
an  iron  hydraulic  fall  tube  six  feet  in  diameter.  At  a  distance 
of  150  feet  below  the  top  of  this  tower,  and  about  half-way 
down  the  side  of  the  hill  a  power  house  (see  Fig.  85)  has 
been  constructed,  into  which  the  termination  of  this  hydraulic 
fall  tube  is  brought.  The  water  from  the  upper  level  conveyed 
by  the  hydraulic  canal  falls  down  this  hydraulic  main,  and  the 
tube  has  a  capacity  for  delivering  100  cubic  feet  of  water  per 
second  at  a  pressure  equal  to  that  due  to  a  height  of  150  feet, 
which  is  about  equal  to  60  Ib.  on  the  square  inch.  The  end 
of  this  hydraulic  tube  terminates  in  three  lateral  branches, 
each  three  feet  in  diameter,  and  closed  with  a  hydraulic  valve 
capable  of  resisting  the  enormous  pressure  of  the  water  above. 
Each  branch  of  the  main  also  sends  off  three  subsidiary 
branches,  which  are  led  to  three  Girard  turbines  or  hydraulic 
motors.  These  machines  are  practically  water  engines,  in 
which  the  flow  of  water  is  made  to  cause  the  revolution 


ELECTRIC   DISTRIBUTION. 


229 


of  a  wheel  with  curved  blades  which  is  enclosed  in  an  iron 
case.  In  one  room  of  the  power  house  there  are  nine  of 
these  turbines,  six  of  350  H.P.  and  three  of  50  H.P.  The 
water  that  passes  through  these  turbines  is  delivered  into  a 
tailrace,  and  then  returns  back  again  into  the  Tivoli  Falls 
at  a  lower  level.  Coupled  to  each  of  those  turbines  is  a  Ganz 
alternating-current  dynamo.  Six  of  these  dynamos  are  of 
350  H.P.  each,  and  can  famish  an  alternating  current  of  45 


FIG.  86. — Turbine  and  Excitor  Dynamo  in  the  Tivoli  Station.      Tivoli- 
Rome   System. 

amperes  at  a  pressure  of  5,000  volts  ;  three  of  the  dynamos 
are  direct-current  machines  of  50  H.P.,  and  can  furnish  a 
continuous  current  of  150  amperes  at  a  pressure  of  125 
volts.  These  smaller  dynamos  are  employed  for  furnish- 
ing the  current  required  to  magnetise  the  field  magnets  of 
the  larger  machines.  A  view  of  one  of  these  smaller  machines 


230     ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

and  its  associated  turbine  is  shown  in  Fig.  86.  These  six 
large  alternators  send  their  current  to  a  switchboard  which 
is  60  feet  long  and  15  feet  high.  From  the  power  house 
proceed  four  stranded  copper  cables,  each  consisting  of  19 
copper  wires,  the  over-all  diameter  of  each  cable  being  about 
one  inch.  Each  cable  weighs  980  kilogrammes  per  kilo- 
metre, or  nearly  two  tons  per  mile.  These  cables  convey 
the  current  across  the  Campagna  from  Tivoli  to  Eome,  a 
distance  of  about  18  miles,  and  in  all  four  lines  there  is  a 
total  weight  of  about  120  tons  of  copper.  These  four  cables 


FIG.  87. — The  Half-way  House.    Tivoli-Rome  Electric  Lighting  System. 

are  supported  by  porcelain  insulators,  which  are  carried  upon 
crossbars  on  iron  posts,  placed  at  intervals  throughout  the 
whole  length  of  the  line.  These  porcelain  insulators  are 
similar  to  those  used  for  carrying  telegraphic  lines,  with  the 
addition  that  they  contain  an  arrangement  for  keeping  the 
edges  of  the  insulator  moistened  with  a  highly  insulating 
oil.  These  oil  insulators  have  been  designed  specially  with 
the  object  of  insulating  the  line  for  the  5,000  volts  pressure, 


ELECTEIG   DISTRIBUTION. 


231 


Half-way  between  Tivoli  and  Eome  there  is  a  half-way 
house  (see  Fig.  87),  in  which  dwells  a  custodian  whcse  duty 
it  is  to  inspect  the  whole  of  the  line.  The  power  station 
at  Tivoli  provides  in  all  a  total  of  2,000  H.P.  in  the  form 
of  an  electric  current  at  a  pressure  of  5,000  volts.  The 
copper  transmission  cables  are  made  of  hard-drawn  copper 
wire,  and  of  a  cross-section  of  100  square  millimetres. 
The  poles  carrying  the  cables  are  placed  35  metres  apart, 


jMSn 

mS®:? 


FIG.  88. — View  of  the  Switchboard  Arrangements  in  the  Porta  Pia  Trans- 
former House.     Tivoli-Rome  System. 

in  almost  a  straight  line  across  the  Campagna,  and 
there  are  707  poles  altogether  between  Tivoli  and  Eome. 
The  resistance  of  each  cable  is  about  38  ohms  for  the 
eighteen  miles.  Each  cable  can  convey  100  amperes,  and 
the  four  cables — that  is,  the  two  pairs — can  therefore  con- 
vey 200  amperes  from  Tivoli  to  Eome,  which  is  equal  to 
the  output  of  five  of  the  large  alternators.  In  the  passage 
from  Tivoli  to  Borne  the  current  undergoes  a  fall  in  pressure 


232    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

due  to  the  resistance  of  the  line,  which  is  about  800  ohms  in 
the  eighteen  miles  when  working  at  full  load.  The  cables, 
therefore,  dissipate  about  16  per  cent,  of  the  whole  of  the  power 
capable  of  being  transmitted  at  full  load.  The  alternators 
have  been  so  constructed  that  they  can  all  work  in  parallel, 
sending  their  current  into  one  or  both  pairs  of  cables  as  may 
be  desired.  On  arriving  at  Kome  the  current  enters  a 
transformer  house  (see  Fig.  88)  placed  just  outside  the  Porta 
Pia  of  Rome.  In  this  transformer  house  are  placed  a 
series  of  transformers,  sixteen  in  number,  each  capable 
of  transforming  30,000  watts  from  5,000  to  2,000  volts.  This 
transformer  house  is  a  substantially-built  stone  building 
two  stories  in  height.  The  current,  reduced  from  5,000  to 
2,000  volts,  is  then  transmitted  by  underground  cables  through 
the  streets  of  Rome  to  certain  transformer  centres,  where  it  is 
again  reduced  in  pressure  to  100  volts.  In  addition  to  this,  a 
portion  of  the  current,  at  a  pressure  of  2,000  volts,  is  utilised 
for  working  a  series  of  alternating  current  arc  lamps.  There 
are  six  circuits  of  these  arc  lamps,  each  circuit  being  arranged 
for  48  lamps.  Each  series  of  lamps  takes  a  current  of  14 
amperes,  and  there  is  a  special  automatic  pressure-regulator 
attached  to  the  transformer  supplying  these  arc  light  circuits, 
so  that  the  current  is  kept  perfectly  constant,  whatever  may  be 
the  number  of  lamps  upon  the  circuit.  In  addition  to  the  arc 
light  circuits,  there  are  five  or  six  incandescent  lighting  circuits 
consisting  of  Siemens  concentric  steel-armoured  cables  laid 
underground.  The  current  supplied  from  Tivoli  can  be 
arranged  to  assist  the  Cerchi  station,  so  that  the  two  stations, 
18  miles  apart,  one  in  the  gasworks  and  the  other  at  Tivoli, 
assist  one  another  in  furnishing  current  for  incandescent 
and  arc  lighting  in  Rome  as  may  be  required.  This  Tivoli 
station  is  one  of  the  finest  examples  of  an  alternating-current 
station  operated  by  water-power,  and  it  has  been  in  perfectly 
successful  operation  since  the  year  1890.  In  order  to  protect 
the  overhead  line,  going  over  18  miles  along  the  Campagna, 


ELECTEIC  DISTRIBUTION. 


233 


from  damage  from  lightning  there  are  lightning  protectors 
of  the  kind  described  on  page  157,  placed  at  Tivoli,  in  the 
transformer  house  at  Rome,  and  in  the  half-way  house  shown 
in  Fig.  87.  These  protectors  have  proved  sufficient  to  guard 
the  dynamos  and  transformers  from  danger  by  lightning 
stroke. 

The  general  arrangement  of  a  low-pressure  continuous  cur- 
rent station  on  the  three -wire  system  may  be  described  as 
follows :  In  the  station  are  placed  a  series  of  dynamos,  each 
dynamo  in  most  English  practice  being  direct-coupled  to  a 
high-pressure  compound  steam  engine.  In  Fig.  79  is  shown 


FIG.  89. — Diagram  showing  the  arrangement  of  Dynamos  D  D,  and 
Lamps  L  on  the  "  Three-wire  "  System  of  Distribution. 

such  a  steam  dynamo,  in  which  the  armature  shaft  is  coupled 
direct  to  the  crank  shaft  of  a  Willans  engine.  This  combina- 
tion of  engine  and  dynamo  on  the  same  bedplate  is  obviously 
very  economical  in  floor  space,  and  has  therefore  naturally 
becoma  very  popular  in  places  where  space  is  valuable. 
A  series  of  these  engine-dynamo  sets  is  generally  arranged 
in  a  central  station,  each  set  being  either  similar  in  size 
and  appearance,  or  else  in  graduated  sizes.  In  Fig.  91 
is  shown  the  interior  of  the  St.  Pancras  electric  lighting 
station,  and  in  Fig.  92  the  interior  of  the  central  station  at 
Glasgow.  In  each  of  these  the  steam  dynamos  are  arranged 
to  generate  current  generally  at  either  220  volts  or  440  volts, 
delivering  this  current  to  a  switchboard,  If  the  current  is 


234    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

supplied  direct  to  the  circuits,  then  the  dynamos  are  joined  in 
pairs  between  the  three  bars  of  the  switchboard,  as  shown  in 
Figs.  89  and  90.  From  fche  ends  of  these  "omnibus  bars," 
as  they  are  called,  are  laid  out  a  series  of  feeding  mains  or 
feeders,  which  feeders  terminate  in  the  distributing  mains  laid 
down  in  the  streets  of  the  district  to  be  supplied.  The  circuits 
of  the  various  houses  are  connected  up  to  these  triple  distribut- 
ing mains  in  such  a  manner  that  as  nearly  as  possible  one-half 
of  the  lamps  are  joined  in  between  one  pair  of  mains  and  the 


Fia.  90. — Dynamos  in  a  Three-wire  Low-Pressure  Station. 

other  half  between  the  other.  In  laying  out  such  a  district 
great  discretion  and  knowledge  have  to  be  brought  to  bear  in 
selecting  points,  which  are  called  the  feeder  centres,  so  that  at 
those  points  the  pressure  may  be  kept  as  constant  as  possible. 
Each  dynamo  in  the  central  station  may  be  regarded  as  a 
pump,  pumping  its  current  into  the  positive  main  omnibus 
bar,  from  which  it  flows  out  by  the  feeders  to  the  distributing 
circuit  and  returns  back  again  to  the  opposite  main.  During 
the  hours  of  heavy  demand  for  current  there  is,  of  course,  a 


ELECTRIC   DISTRIBUTION.  235 

large  current  flowing  out  by  these  feeders,  and  a  corres- 
pondingly large  fall  of  pressure  down  the  feeder.  Hence  the 
pressure  at  the  terminals  of  the  dynamos  has  to  be  kept  up  in 
order  to  allow  for  this  fall.  It  is  usual  to  maintain  the  pres- 
sure at  certain  feeding  points  constant  at  100  or  110  volts, 
whatever  may  be  the  nature  of  the  supply,  and  to  do  this  the 
pressure  at  the  dynamo  terminals  has  to  be  varied  from  110  to 
150  volts  or  thereabouts,  as  the  demand  varies  at  different 
hours  of  the  day  or  night.  It  will  thus  be  seen  that  in 
the  feeders  in  a  low-pressure  station  the  loss  of  energy  is 
greatest  at  the  time  of  greatest  load,  because  they  are  then 
traversed  by  the  largest  currents,  and,  therefore,  the  total 
supply  of  energy  from  the  dynamos  has  to  be  equal  to  the 
amount  required  by  the  lamps  in  use  plus  that  required  to 
supply  the  corresponding  loss  in  the  feeders.  A  low-pressure 
continuous  current  station  has,  consequently,  its  least  efficiency 
of  distribution  at  the  time  of  fullest  load.  Exactly  the  opposite 
is  the  case  with  the  alternating  current  supply.  In  this  case 
the  efficiency  of  distribution  is  a  maximum  at  the  time  of 
largest  demand. 

We  may  take  as  a  good  example  of  a  low-pressure  station 
the  St.  Pancras  Vestry  Electric  Lighting  Station,  which 
has  been  carried  out  by  the  local  authority  of  the  parish 
of  St.  Pancras,  in  the  north-west  of  London,  for  the  supply 
of  electrical  energy  for  lighting  and  motive  power  purposes 
in  that  district.  In  this  part  of  London,  which  is  densely 
populated,  it  was  considered  advisable  to  adopt  the  continuous 
current  low-pressure  system,  and  a  typical  station  of  this  kind 
was,  therefore,  designed  for  the  district  by  Prof.  Henry 
Eobinson.  The  following  is  a  description  of  the  works  as 
given  by  him :  The  station  buildings  consist  of  an  engine- 
house  106ft.  by  26ft.,  a  boiler-house  53ft.  by  35ft.,  a  coal 
store  43ft.  by  lift.,  a  battery  room  40ft.  by  14ft.  6in.,  as 
well  as  a  testing-room,  office,  stores,  and  an  underground 


236    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

tank  for  condensing  water  capable  of  containing  170,000 
gallons  of  water,  and  a  chimney  shaft  5ft.  square  inside 
and  90ft.  high.  The  dynamo  room  contains  eleven 
engines  and  dynamos,  which  are  erected  on  a  concrete 
foundation,  surrounded  by  sand  to  prevent  vibration  being 
communicated  to  the  walls.  The  floor  is  carried  indepen- 
dently of  the  engine  foundations  by  cantilevers  from  the 
walls.  The  engines  employed  are  of  the  Willans  compound 


FIG.  91. — View  of  the  Interior  of  St.  Pancras  Electric  Lighting  Station, 
London,  showing  the  Dynamos. 

central  valve  type,  and  the  dynamos  are  of  the  6-pole  Kapp 
type  made  by  Messrs.  Johnson  and  Phillips.  A  view  of  the 
interior  of  this  station,  showing  the  long  perspective  of 
engines  and  dynamos,  is  given  in  Fig.  91.  Each  Willans 
engine  is  coupled  direct  to  its  corresponding  dynamo.  These 
dynamos  furnish  a  continuous  current,  nine  of  them  giving 
680  amperes  at  a  pressure  varying  from  112  to  130  volts,  and 
three  of  them  giving  145  volts,  with  a  small  current  for 
charging  secondary  batteries  at  a  distant  sub-station.  The 


ELECTRIC   DISTRIBUTION.  237 

remaining  two  dynamos  are  wound  for  an  output  of  90 
amperes  at  540  to  575  volts.  These  are  used  for  working 
the  street  arc  lamps  as  well  as  for  charging  in  series  four 
sets  of  storage  batteries  at  the  central  station  which  are 
capable  of  discharging  at  a  rate  of  60  to  75  amperes.  The 
boiler  plant  consists  of  five  Babcock-Wilcox  boilers,  each 
capable  of  evaporating  over  5,0001b.  of  water  per  hour,  the 
working  pressure  being  1701b.  on  the  square  inch.  Along 
the  top  of  these  runs  a  steam  main  into  which  all  the  boilers 
deliver  their  steam  and  from  which  the  engines  separately 
take  their  steam.  The  steam,  having  done  its  work  in  the 
engines,  is  then  either  delivered  into  the  atmosphere  or  passes 
into  a  jet  condenser,  which  draws  its  water  from  the  bottom 
of  the  underground  tank.  From  the  top  of  this  tank  the  hot 
water  is  pumped  by  independent  pumps  to  a  cooling  appa- 
ratus, which  is  capable  of  dealing  with  a  minimum  of  10,000 
gallons  of  water  per  hour.  The  cooling  arrangement  consists 
of  a  large  surface  of  corrugated  sheet  iron  attached  to  a 
framing  placed  round  the  chimney.  The  water,  after  flowing 
along  the  corrugated  surface,  is  collected  underneath  and 
returned  to  the  bottom  of  the  tank.  There  is  also  an  air 
pump  in  the  station  for  delivering  dry  air  into  the  street  main 
culverts  at  the  rate  of  5,000  cubic  feet  of  air  per  hour. 

The  street  lamps  arc  Brockie-Pell  arc  lamps,  placed  on  tall 
iron  posts  placed  in  the  centre  of  the  road  where  admissible. 
They  are  fixed  at  distances  varying  from  160ft.  to  245ft. 
apart,  the  height  of  the  lamp  varying  from  22ft.  to  25ft. 
above  the  pavement.  These  lamps  are  worked  11  in  series 
near  the  central  station  and  10  in  series  at  a  distance,  and 
each  lamp  takes  a  current  of  10  amperes,  which  is  sup- 
plied by  the  two  500-volt  dynamo  machines.  The  main 
switchboard  for  the  incandescent  lighting  is  arranged  on  the 
"  three-wire"  system.  Each  dynamo  is  provided  with  a 
double-pole  switch  and  copper  coupling  strips  to  connect  the 


238    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

machines  to  the  third  or  middle  wire  and  to  the  different  bars 
on  the  feeder  board  on  either  side  of  the  circuit.  There  is  an 
amperemeter  on  each  dynamo  to  indicate  the  current  being 
given  out.  On  the  switchboard  there  are  four  positive 
omnibus  bars  and  four  negative  bars,  and  at  full  load  one 
machine  can  be  run  on  to  each  bar  and  be  worked  at  any 
pressure  which  may  be  necessary  to  serve  the  feeders  which 
are  switched  on  to  it.  Thus  the  dynamos  may  be  worked  at 
different  pressures  to  suit  the  demand  in  the  districts  which 
they  serve. 

There  are  seven  feeder  mains  going  out  of  the  station,  in 
addition  to  a  direct  supply  to  the  distributing  mains.  The 
mains  are  laid  throughout  the  district  on  the  three-wire 
system,  the  principal  mains  being  sufficient  to  supply  25,000 
incandescent  lamps  of  16 -can die  power  in  use  simultaneously. 
These  conductors  are  laid  down  under  the  streets,  and  are 
composed  of  copper  strips  1  Jin.  wide  and  |in.  thick,  supported 
on  edge  in  glazed  porcelain  insulators,  which  are  carried  on 
small  cast-iron  brackets  built  into  the  walls  of  the  culverts. 
Some  of  the  mains  are  cables  laid  in  cast-iron  pipes,  and  some 
of  them  are  armoured  cables.  In  addition  to  this,  the  current 
is  supplied  from  the  central  station  to  a  distant  secondary 
battery  station  1,140  yards  away,  which  contains  two  batteries 
of  58  E.P.S.  cells  each.  By  varying  the  number  of  dynamos 
in  use  at  the  station  and  the  number  of  feeders  in  connec- 
tion with  the  omnibus  bars,  the  engineers  in  charge  of  the 
station  have  it  in  their  power  to  regulate  the  electric  pressure 
between  the  distributing  mains  laid  down  in  the  streets,  and 
the  supply  is  so  regulated  as  to  keep  the  pressure  between  the 
middle  main  or  wire  and  each  of  the  outer  wires  to  110  volts. 

A  very  similar  station  was  established  for  the  supply  of 
electric  current  in  Glasgow  by  the  Glasgow  Corporation.  A 
view  of  part  of  the  interior  of  this  station  is  shown  in  Fig.  92. 


ELECTRIC   DISTRIBUTION. 


UNIVERSITY 
^ 


The  station  buildings  are  substantially  built,  standing  on  a 
layer  of  concrete  two  feet  thick  laid  throughout  the  entire  block. 
These  buildings  consist  of  an  engine  and  dynamo  room,  boiler 
room,  workshop,  stores,  and  offices.  The  boiler  room  con- 
tains six  steel  boilers  of  the  marine  type,  measuring  12ft.  by 
10ft.  in  diameter,  and  working  at  a  pressure  of  1601b.  Each 
contains  two  furnaces  of  the  corrugated  type,  with  an  internal 
diameter  of  3ft.  These  boilers  deliver  steam  through  a  ring 


FIG.  92. — View  of  Part  of  the  Glasgow  Corporation  Electric  Lighting 

Station. 


main  into  the  engine  room.  The  engine  room  contains  a  series 
of  Willans  engines  coupled  to  dynamos  (see  Fig.  92).  The 
engines  are  Willans  central  valve  compound  engines,  and  the 
nine  dynamos  are  two-pole  shunt-wound  machines  with 
drum  armatures.  The  boilers  are  fed  with  gas  coke  obtained 
from  the  Corporation  gas  works.  Over  the  boiler  house  is 
placed  an  accumulator  room  containing  114  secondary 
battery  cells.  Each  cell  contains  61  lead  plates,  and  has  a 
capacity  for  storing  up  and  discharging  1,000  ampere-hours 


240     ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

of  electrical  quantity.  The  current  from  the  dynamos, 
which  is  generated  at  from  210  to  230  volts,  is  led  to 
a  distributing  board  in  the  station,  and  this  supply  has 
a  three-wire  system  of  distribution  laid  down  in  the 
streets.  The  three  copper  conductors  consist  of  copper 
strip,  which  is  laid  on  porcelain  insulators  contained  in  iron 
culverts.  The  current  is  brought  to  the  two  outer  wires  of 
the  three-wire  strip  by  conductors,  which,  as  in  the  St. 
Pancras  station  before  described,  are  called  "  feeders,"  and 
which  consist  of  only  two  mains,  a  positive  and  a  negative 
main.  The  lamps  are  all  joined  in  parallel  between  the 
middle  main  of  the  distributing  mains  and  one  of  the  outer 
ones,  the  total  of  about  46,000  eight  candle-power  lamps  being, 
as  far  as  possible,  connected  so  that  one-half  of  the  lamps  are 
ioined  in  between  the  middle  main  and  one  of  the  others, 
called  the  "positive,"  and  the  other  half  of  the  lamps  being 
connected  in  between  the  middle  main  and  the  other  conductor, 
called  the  "negative."  The  middle  wire,  therefore,  serves  to 
carry  the  balance  of  current  from  one  part  of  the  distributing 
system  to  the  other  at  those  times  when  the  number  of  lamps 
joined  in  between  the  two  sides  of  the  three-wire  system  is  not 
exactly  equal. 

These  two  stations  may  be  taken  as  fairly  typical  examples 
of  low-pressure  stations  as  established  in  Great  Britain, 
although  many  other  equally  good,  such  as  the  Kensington 
and  Knightsbridge  and  Westminster  stations  in  London, 
might  be  described.  Such  brief  space  as  we  are  able  to 
give  to  this  portion  of  the  subject  in  no  way  enables  us 
to  exhaust  the  description  of  the  whole  of  the  methods  for  a 
direct  current  low-pressure  supply ;  but  the  general  tendency, 
at  any  rate  in  England,  is  to  adopt  the  direct-driven  type 
of  dynamo,  on  account  of  the  considerable  economy  which 
is  thereby  effected  in  space.  In  some  low-pressure  stations, 
where  space  is  not  of  much  importance,  it  is  found  convenient 


ELECTRIC   DISTRIBUTION. 


241 


to  drive  the  dynamos  by  means  of  belts  from  the  flywheels 
of  engines. 


In  Fig.  93  is  shown  a  portion  of  the  interior  of  a  Con- 
tinental electric  lighting  station — that  of  the  City  of  Brussels. 


242    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

This  station  is  constructed  to  contain  plant  of  3,000  indi- 
cated horse-power,  the  unit  adopted  being  a  500  horse-power 
engine  driving  two  250  horse-power  dynamos.  Provision 
is  made  for  placing  six  such  units,  one  being  for  reserve. 
The  boiler  plant  in  1894  consisted  of  three  Babcock-Wilcox 
water-tube  boilers  capable  of  evaporating  7,5001b.  of  water 
per  hour.  The  engine  room  has  space  for  six  or  more, 
of  the  horizontal  compound  condensing  type.  The  low- 
pressure  cylinders  are  40  inches  in  diameter  and  the 
high-pressure  cylinders  26  inches  in  diameter,  the  length  of 
the  stroke  being  4ft.  The  speed  is  capable  of  variation 
between  62  and  75  revolutions  per  minute.  Each  engine  is 
fitted  with  two  grooved  flywheels  20  feet  in  diameter,  and 
weighing  20  tons.  These  drive  the  dynamos  attached  to  each 
engine  by  means  of  ropes.  The  dynamos  shown  in  the 
illustration  are  of  the  four-pole  type,  shunt-wound,  with 
drum  armatures,  and  with  an  output  capacity  of  145  kilowatts. 
These  dynamos  supply  a  three-wire  distributing  system,  the 
cables  being  laid  down  underground  in  iron  pipes. 

In  addition  to  the  generating  plant  a  storage  plant  of 
accumulators  is  provided,  in  two  batteries  of  70  cells 
each.  Each  battery  is  capable  of  giving  a  normal  discharge 
of  350  amperes  for  ten  hours.  The  underground  cables 
consist  of  copper- stranded  cables  insulated  with  india- 
rubber,  which  are  drawn  into  iron  pipes.  This  work  was 
designed  and  carried  out  by  the  engineers  of  the  Silver- 
town  Company. 

The  above  descriptions  of  alternating  current  and  con- 
tinuous current  generating  stations  will  be  sufficient  to  give 
the  student  a  general  view  of  the  methods  which  are  at 
present  (1899)  employed  for  the  generation  of  electric  current 
for  illuminating  purposes.  It  would  lead  us  into  matters 
too  highly  technical  to  attempt  to  discuss  fully  when,  and 


ELECTRIC  DISTRIBUTION.  243 

under  what  conditions,  each  system  finds  its  best  appli- 
cation. Suffice  it  to  say  that  no  one  system  can  be  described 
as  the  best  system  ;  both  the  low-pressure  direct  current  and 
the  high-pressure  alternating  current  systems  have  certain 
peculiar  advantages  and  disadvantages,  which  have  to  be 
considered  in  the  designs  for  a  system  of  electric  distri- 
bution. The  engineer  who  is  called  upon  at  the  present 
time  to  design  and  carry  out  a  system  of  electric  supply  for 
public  purposes  has  to  consider  a  large  number  of  facts 
which  determine  the  choice  that  shall  be  made  of  the  method 
of  supply.  The  tendency  at  the  present  time  is  to  employ 
alternating  currents  in  the  transmission  of  the  power  from  a 
distance,  and  to  transform  them  into  continuous  currents  in 
the  region  of  supply. 

Taking  such  cases  as  the  central  areas  in  large  towns,  it 
will  be  found  that  at  the  present  time  the  demand  for  lighting 
will  vary  from  one  8-c.p.  lamp  to  three  8-c.p.  lamps  per  yard 
of  distributing  main  or  of  house  frontage.  In  the  outer 
portions  of  large  towns  and  in  smaller  country  ones  the 
demand  for  light  is  much  more  scattered,  and  in  these  cases 
the  only  possible  system  of  supply  which  meets  the  conditions 
is  the  alternating  current  .system.  It  is  possible  to  some 
extent  to  combine  together  the  advantages  of  the  two  sy terns. 
The  generating  plant  in  the  station  can  be  made  to  furnish  a 
continuous  current  at  a  low  pressure.  This  can  be  used  to 
supply  the  district  within  a  mile  or  two  miles  round  the 
station  with  current  furnished  directly  from  the  generating 
machines.  Alternating  currents  can  then  be  used  to  transmit 
power  to  the  distant  portions  of  the  area  of  supply,  and  the 
pressure  can  then  be  reduced,  either  in  sub-centres  or  in  the 
houses,  by  means  of  transformers,  to  the  normal  pressure  of 
100  volts. 

In  every  case  in  which  an  engineer  is  called  upon  to  advise 
upon  the  selection  of  a  system  of  electric  lighting  for  a  town, 

u  2 


244   ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

he  has  to  examine  carefully  the  conditions  which  prevail, 
in  order  to  guide  himself  in  making  a  decision  whether  to 
employ  an  alternating  current  or  a  continuous-current  system. 
If  continuous-current  is  employed  it  may  be  generated  either 
at  high  or  at  low  pressure.  In  the  case  of  the  low-pressure 
continuous  current  stations,  the  generating  machines  are 
now  generally  designed  to  give  a  voltage  from  400  to  460 
volts.  These  machines  supply  the  outer  wires  of  a  three- 
wire  system  of  distribution,  and  the  customers'  houses  are 
wired  for  200  or  210  volt  lamps.  The  lamps  are,  as  far  as 
possible,  divided  equally  between  the  two  sides  of  the  three- 
wire  mains,  but,  since  it  is  impossible  at  all  times  to  preserve 
a  perfect  balance  in  the  total  number  of  lamps  on  either 
side  of  the  middle  wire,  it  follows  that,  unless  compensation 
is  made  in  some  way,  the  current  per  lamp  will  be  greatest 
on  that  side  which  has  the  fewest  lamps,  and  will  tend  to 
destroy  them  if  it  is  above  the  normal  current.  Hence  some 
means  has  to  be  found  to  equilibrate  the  current  per  lamp, 
or,  which  comes  to  the  same  thing,  render  equal  the  voltage 
on  each  side  of  the  middle  main.  This  is  generally  done  by 
means  of  secondary  batteries,  with  or  without  balancing 
dynamos. 

The  secondary  battery,  as  now  employed,  consists  of  a 
series  of  cells,  either  glass  vessels  or  lead-lined  boxes,  in 
which  are  a  double  set  of  lead  plates.  These  plates  are 
placed  parallel  to  each  other  and  slightly  separated.  Alternate 
plates  are  connected  together  by  a  lead  bar,  and  are  called 
respectively  the  positive  and  the  negative  sections.  The  lead 
plates  now  used  are  generally  either  pasted  plates  or  formed 
plates.  The  pasted  plates  consist  of  lead  grids  which  form 
a  reticulated  lead  plate,  the  interstices  or  holes  being  filled 
in  with  a  paste  or  active  material,  made  by  mixing  with 
sulphuric  acid,  red  lead  or  litharge.  The  positive  sections 
are  generally  made  with  red  lead  and  the  negative  with 


ELECTRIC  DISTRIBUTION.  245 

litharge.  The  cells  are  filled  with  an  electrolyte  composed 
of  nine  parts  of  water  and  one  of  sulphuric  acid,  or,  in  some 
cases,  four  parts  of  water  and  one  of  sulphuric  acid  by  volume. 
When  the  cell  has  a  current  passed  through  it  from  the 
positive  to  the  negative  plates,  the  current  breaks  up  the 
sulphate  of  lead  which  is  formed  as  a  result  of  the  action 
of  the  sulphuric  acid  upon  the  oxide  of  lead,  and  finally  con- 
verts the  active  material  on  the  positive  plate  into  peroxide 
of  lead,  and  the  active  material  on  the  negative  plate  into 
reduced  or  spongy  lead.  This  being  the  case,  there  is  produced 
an  electromotive  force,  and  the  cell  contains  chemical  potential 
energy  stored  up  in  it  which  can  be  transformed  into  the 
energy  of  an  electric  current.  Generally  speaking,  from  8  to 
16  watt-hours  of  energy  per  pound  of  plates  (positives  and 
negatives  together)  may  be  stored  up.  The  electromotive  force 
of  each  cell  when  so  charged  is  about  2  volts.  To  charge  the 
cell  requires  an  impressed  electromotive  force  of  2-5  volts. 
The  useful  quantity  of  electricity  which  can  be  stored  up  in 
a  secondary  cell  varies  from  4  to  8  ampere-hours  per  pound 
of  plates  according  to  construction.  During  the  process  of 
discharge  the  chemical  difference  of  the  plates  diminishes, 
each  plate  being  more  or  less  converted  into  a  white  sub- 
stance, called  sulphate  of  lead.  In  the  course  of  the 
discharge  the  sulphuric  acid  is  taken  up  out  of  the  dilute 
acid,  and  hence  the  density  of  the  electrolyte  diminishes. 
In  the  process  of  charging,  the  sulphate  of  lead  on  both 
plates  is  broken  up  again  and  the  density  of  the  dilute  acid 
or  electrolyte  increases. 

In  certain  types  of  plates,  called  Plante  plates,  the  active 
material  is  not  put  upon  the  plates,  but  is  formed  out  of  them 
by  a  process  of  treating  the  plates.  The  plates  are  con- 
structed by  building  them  up  of  lead  strips,  or  in  other  ways, 
so  as  to  expose  a  very  large  surface  of  lead  to  the  acid.  The 
surface  of  one  set  of  plates  is  then  converted  into  peroxide 


246    ELECTRIC  LAMPS  AND  ELECTEIC  LIGHTING. 

of  lead,  and  the  surface  of  the  other  set  of  plates  more  or 
less  into  spongy  lead  by  subjecting  them  to  a  process  called 
"forming,"  which  consists  in  sending  a  current  first  one 
way  through  the  cell,  and  then  in  the  other  direction,  with 
intervals  of  rest.  The  plates  are  formed  in  some  electrolyte 
which  has  a  strong  oxidising  re- action  upon  lead,  and  for 
this  purpose  nitric  acid  may  be  used  in  and  with  dilute 
sulphuric  acid  in  certain  proportions. 

There  are  many  varieties  of  secondary  cells,  differing  in 
details,  but  the  three  principal  types  in  general  use  may  be 
said  to  be  the  pasted  or  packed  grid  type,  as  made  by  the 
Electrical  Power  Storage  Company,  the  modified  Plante  or 
formed  plate,  as  made  by  the  D.P.  Battery  Company,  and  an 
intermediate  class,  called  the  Chloride  storage  cell,  made  by 
the  Chloride  Electrical  Storage  Syndicate.  This  latter  cell 
is  so  called  ^because  the  plates  are  prepared  by  first  casting 

tablets  of  a  mixture  of  zinc  and  lead  chlorides.     These  are 

« 

held  in  position  in  a  lead  grid  cast  round  them,  formed  of 
antimony  and  lead.  These  composite  plates,  as  thus  formed, 
are  not,  however,  capable  of  being  used  as  storage  cell  plates. 
The  plates  so  made  are  placed  in  a  bath  of  chloride  of  zinc 
and  acted  upon  electro-chemically  by  being  coupled  to  a  zinc 
plate.  The  final  result  of  this  process  is  to  leave  the  tablet 
in  the  form  of  spongy  lead,  or  lead  in  a  very  finely  divided 
condition.  The  plates  that  are  to  form  the  positive  plates  in 
the  storage  cell  then  have  their  spongy  lead  converted  into 
peroxide  of  lead  by  the  action  of  an  electric  current  passing 
through  a  solution  of  sulphuric  acid  in  which  the  plates  to  be 
formed  are  placed  with  companion  plates  of  lead.  The  result 
of  the  operation  is  to  produce  two  sets  of  plates  consisting  of 
lead  grids,  one  set  consisting  of  spongy  lead  tablets  held  in  the 
grid  apertures,  and  the  other  set  consisting  of  peroxide  of  lead 
blocks  fixed  in  the  same  manner.  Sets  of  the  positive 
(peroxide  of  lead)  and  negative  (spongy  lead)  plates  are  placed 


ELECTRIC  DISTBIBUTION.  247 

in  a  glass  vessel  in  dilute  sulphuric  acid,  the  plates  being 
arranged  so  that  alternate  plates  are  identical  and  are 
connected  together  by  a  lead  band  welded  to  them. 

This  arrangement  constitutes  a  secondary  cell,  and  a  series 
of  cells  connected  together  is  called  a  secondary  battery.  The 
plates  in  each  cell  are  prevented  from  touching  each  other  by 
separators  of  ebonite  or  glass,  and  are  held  off  the  bottom  of 
the  cell  by  ebonite  bars. 

One  great  defect  of  the  original  packed  grid  type  of  plate 
was  a  tendency  for  the  active  material  to  fall  out  of  the 
interstices  of  the  lead  plate.  This  has  been  greatly  remedied 
by  careful  manufacture.  In  some  types  of  packed  plate  the 
active  material  is  kept  on  the  plate  by  enclosing  the  plate  in 
a  jacket  made  of  perforated  celluloid.  This  allows  access  of 
the  electrolyte,  but  tends  to  prevent  the  falling  out  of  material. 
The  plates  can  then  be  placed  much  closer  together,  and  a 
compact  cell  obtained  which  is  useful  for  traction  purposes. 
In  some  cases  the  active  material  is  pressed  into  grooves 
pressed  or  cast  in  the  lead  plate. 

In  all  cases  in  which  storage  batteries  are  employed  in 
connection  with  generating  stations,  and  where  weight  and 
space  are  of  secondary  importance,  it  is  advisable  to  employ 
plates  substantially  made  and  separated  by  a  considerable 
distance.  The  object  being  to  avoid  short  circuiting  by  reason 
of  active  material  dropping  out  and  connecting  adjacent 
plates,  and  also  to  allow  free  circulation  of  fresh  electrolyte 
between  the  plates. 

A  secondary  battery  in  a  generating  station  is  set  up  in  a 
dry  and  well  ventilated  room.  The  cells  are  placed  on 
wooden  trays  filled  with  sawdust,  each  tray  being  sup- 
ported on  four  glass  insulators  having  oil  in  them  to 
prevent  surface  leakage.  The  cells  are  carried  on  stout 
wooden  stands  in  two  or  more  tiers,  and  are  connected  in 


248    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

series  with  one  another.  In  the  case  of  a  three-wire  system 
of  distribution  two  batteries  of  cells  are  employed,  one  on 
each  side  of  the  middle  wire. 


If,  for  instance,  the  system  is  a  three-wire  system  supplying 
customers  at  200  volts  on  each  side,  then  it  is  usual  to  put  in 
110  or  120  cells  on  each  side  between  the  middle  wire  and 
each  outer  main.  In  order  to  charge  these  cells  an  electro- 
motive force  of  2-5  volts  per  cell  is  required.  Hence,  if  the 
cells  are  to  be  charged  at  any  time,  it  is  requisite  to  provide  a 
machine  called  a  "booster,"  the  function  of  which  is  to  raise 
the  circuit  voltage  by  0'5  volt  per  cell.  This  booster  consists 


\(t 


D,  D.— Dynamos.  b,  b,— Batteries.  o,  o.— Boosters. 

B,  B. — Balancers.  I,  1. — Lamps. 

FIG.  94. — Arrangements  for  Three- wire  Distribution. 

of  an  electric  motor  coupled  to  a  dynamo.  The  dynamo 
armature  is  so  wound  that  it  can  pass  without  undue  heating 
the  charging  current  for  the  battery,  and  when  running  it 
adds  an  extra  electromotive  force  in  series  with  that  of  the 
main  dynamos,  and  forces  the  charging  current  into  the  cells. 
When  the  cells  are  charged  the  booster  is  removed  or  stopped, 
and  the  cells  can  then  deliver  up  current  to  the  circuit  after 
tne  main  dynamos  have  been  stopped.  The  arrangements 
usual  in  a  three- wire  continuous  current  station  are  shown 
diagrammatically  in  Fig.  94. 

The  two  batteries  act  as  regulators.      If  more  lamps  are 
switched  on  on  one  side  of  the  three-wire  system  than  on  the 


ELECTEIC  DISTRIBUTION.  249 

other,  the  battery  on  that  side  gives  out  the  necessary  extra 
current  whilst  the  main  dynamos  supply  that  which  is  taken 
by  the  remaining  equal  number  of  lamps  on  both  sides  of 
the  middle  wire. 

Thus,  suppose  there  are  at  any  moment  10,000  8  c.p.  lamps 
on  one  side,  and  9,000  8  c.p.  lamps  switched  on  on  the  other, 
the  main  dynamo  provides  the  current  for  the  9,000  lamps 
on  each  side  of  the  middle  wire,  and  the  battery  on  one 
side  furnishes  the  current  for  the  odd  1,000  lamps  on  its 
own  side. 

Instead  of  the  battery,  balancing  dynamos  are  sometimes 
employed.  These  are  small  dynamos,  either  driven  inde- 
pendently, or  else  having  their  shafts  coupled  together.  In  the 
latter  case  each  machine  operates  either  as  generator  or  motor 
according  as  the  electromotive  force  of  the  machine  is  greater 
or  less  than  the  fall  of  potential  across  the  two  mains  between 
which  it  is  coupled.  If  the  number  of  lamps  on  one  side 
of  the  three-wire  system  exceeds  that  on  the  other,  then 
one  of  the  two  coupled  dynamos  which  is  on  the  same  side 
acts  as  a  generator,  and  the  other  acts  as  a  motor.  Power 
is,  therefore,  taken  from  the  more  lightly  loaded  side  and 
delivered  to  the  other,  and  the  equilibrium  is  restored,  so 
that  the  current  in  the  middle  or  neutral  wire  is  kept  nearly 
at  zero. 

In  cases  where  the  distances  to  be  covered  by  the  feeders 
are  on  an  average  greater  than  a  mile  and  a-half  or  two 
miles,  the  high-pressure  continuous  current  system  is  often 
employed.  In  this  system  the  generating  machines  are 
continuous  current  dynamos  generating  at  1,000  or  2,000 
volts  pressure.  The  current  from  them  is  .taken  by  high- 
pressure  feeders  to  certain  places  called  transformer  stations. 
In  these  stations  there  is  a  machine  (or  machines)  called  a 
continuous  current  transformer.  This  appliance  is  a  motor 


250    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

and  dynamo  combined  in  one.  In  some  cases  there  are  two 
armatures,  each  having  its  own  field  magnets.  The  one 
armature  takes  current  at  1,000  to  2,000  volts  pressure  and 
acts  as  a  motor,  the  other  armature  acts  as  a  dynamo  and 
generates  current  at  100  or  200  volts  pressure.  The  low- 
pressure  current  is  distributed  by  a  two-wire  low-pressure 
distribution  system  of  mains  used  in  private  lighting. 

In  certain  types  of  continuous  current  transformer  the  two 
armature  windings  are  placed  together,  but  insulated  on  one 
iron  core,  and  there  is  but  one  field  magnet.  Each  armature 
winding  has,  however,  its  own  commutator.  Very  ingenious 
automatic  switches  have  been  designed  which  enable  the 
engineer  at  the  generating  station  to  start  and  control  any  of 
the  motor-generators  in  the  distributing  stations. 

Any  discussion  on  electric  lamps  and  their  use  would  be 
incomplete  which  did  not  include  a  brief  reference  to  the 
subject  of  the  measurement  of  electric  energy  by  meter,  as 
applied  in  the  case  of  public  electric  supply.  The  complete 
solution  of  the  commercial  problem  of  distributing  electric 
energy  for  use  and  sale  is  bound  up  with  the  practical  one 
of  the  production  of  a  satisfactory  electric  house  meter. 

Electric  meters  are  broadly  divided  into  two  classes,  called 
respectively  energy  meters  and  quantity  meters.  The  first 
class  measure  electric  energy  passed  through  them  to  any 
apparatus  or  lamps,  the  second  measure  the  total  quantity 
of  electricity  which  has  passed  through  the  meter.  In  so  far 
as  the  supply  companies  or  corporations  are  compelled  to 
furnish  electric  current  at  a  constant  pressure,  it  follows 
that,  for  all  practical  purposes,  the  measurement  of  electrical 
quantity  is  equally  as  good  as  the  measurement  of  electric 
energy,  in  those  cases  where  the  power  taken  up  is  measured 
by  the  product  of  the  values  of  current  and  the  voltage  at 


ELECTRIC  DISTRIBUTION. 


251 


which  it  is  supplied.  In  the  case  of  alternating-current 
circuits,  having  on  them  motors  or  transformers,  or  arc 
lamps,  it  is  necessary  to  employ  energy  meters,  because  in. 


Closed.  Open. 

FIG.  95.— Bastian's  Electrolytic  Meter. 

those  cases  the  power  taken  up  is  not  always  strictly  measured 
by  the  numerical  product  of  the  current  and  the  voltage. 

A  brief  description  may  in  the  first  place  be  given  of  some 
types  of  much  used  energy  and  quantity  meters,     One  of 


252    ELECTEIC  LAMPS  AND  ELECTRIC  LIGHTING. 

the  simplest  forms  of  quantity  meter,  applicable,  however, 
only  to  continuous  current  circuits,  is  Bastian's  electrolytic 
meter.  In  this  meter  (see  Fig.  95)  the  current  entering  ihe 
consumer's  premises  is  made  to  pass  through  a  glass  vessel 
containing  dilute  sulphuric  acid.  The  current  is  led  into 
and  out  of  the  liquid  by  means  of  platinum  plates,  called 
electrodes.  When  the  current  passes  through  the  liquid  it 
chemically  decomposes  it,  liberating  gaseous  oxygen  and 
hydrogen,  which  escape.  The  amount  of  liquid  is  thereby 
reduced,  and  by  means  of  a  gauge  glass  the  quantity  of 
liquid  left  in  the  vessel  can  be  ascertained  at  a  glance. 
Every  ampere-hour  of  electric  quantity  passed  through  the 
electrolyte  causes  the  destruction  of  a  certain  volume  of  the 
liquid,  and  hence  the  gauge  glass  can  be  graduated  to  show 
at  once  the  ampere-hours  which  have  passed  through  the 
meter.  To  prevent  evaporation  from  diminishing  the  volume 
of  the  acid,  the  surface  is  covered  with  a  shallow  layer  of 
paraffin  oil.  This  meter  has  the  great  advantage  of  extreme 
simplicity  of  construction  and,  therefore,  of  cheapness.  It 
has  the  disadvantage  that  it  requires  filling  up  from  time 
to  time  with  water,  and  that  it  is  not,  without  some  com- 
plications, adapted  for  use  with  very  large  currents. 

A  type  of  ampere-hour  meter  in  extensive  use  is  one  in 
which  the  principle  of  the  electric  motor  is  involved,  and  is 
hence  called  a  motor  meter.  Of  this  form  are  the  meters  of 
Chamberlain  and  Hookham,  Ferranti  and  Perry. 

In  the  Chamberlain  and  Hookham  meter,  the  rotating 
portion  consists  of  two  copper  discs  attached  to  a  vertical 
spindle,  and  arranged  so  as  to  rotate  in  a  magnetic  field 
which  is  parallel  to  the  axis  of  the  discs.  The  current  to  be 
measured  passes  in  at  the  centre  of  one  disc  and  out  at  the 
edge,  being  so  conducted  by  immersing  the  disc  in  mercury. 
The  current  passing  radially  through  the  disc  thus  placed  in 


ELECTRIC  DISTRIBUTION. 


253 


a  magnetic  field  at  right  angles  to  its  plane,  causes  it  to 
revolve.  The  associated  disc  also  is  rotated  in  the  field,  and 
is  retarded  by  the  so-called  magnetic  friction  due  to  eddy 
electric  currents  set  up  in  its  mass.  Hence  a  retarding  force 
proportional  to  the  speed  is  produced. 

In  the  diagram  (Fig.  96),  MM  are  permanent  magnets 
which  have  their  poles  in  connection  with  iron  pole  pieces 


FIG.  96. — Chamberlain  and  Hookham  Electric  Meter. 

P  Pj.  There  is  an  intermediate  iron  piece,  L,  between  the 
poles.  This  is  held  down  on  a  copper  ring,  and  forms  a 
mercury  chamber.  In  this  chamber  one  of  the  copper 
discs  revolves.  The  current  passing  through  the  disc 
causes  it  to  rotate  with  a  speed  proportional  to  the  current 
strength.  The  disc  system  is,  however,  retarded  by  the 


254    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

magnetic  friction  with  a  force  proportional  to  the  speed 
of  rotation.  Hence  the  speed  is  proportional  to  the  current, 
and  the  number  of  revolutions  in  a  given  time  to  the 
total  quantity  of  electricity  which  has  in  that  time  passed 
through  the  meter. 

This  meter,  and  others  of  a  similar  kind,  have  the 
advantage  of  very  low  internal  resistance,  and  therefore, 
very  small  internal  consumption  of  power.  They  measure 


FIG.  97. — Ferranti  Electric  Meter. 

directly  the  quantity  of  electricity  passed  through  them,  but  it 
is  indicated  on  the  dials  as  Board  of  Trade  units  used,  on  the 
assumption  that  the  circuit  pressure  is  constant  and  equal  to 
the  certified  standard  electric  pressure  for  the  supply  station. 

Another  ampere-hour  meter  of  the  same  type,  extensively 
used,  is  that  of  Ferranti  (see  Fig.  97).  In  this  meter  also  a 
layer  of  mercury  is  contained  in  a  thin  cavity  over  an  electro- 
magnet. The  current  passing  into  the  house  or  building  using 


ELECTRIC  DISTRIBUTION.  255 

it  is  made  to  flow  through  the  coils  of  the  electromagnet,  and 
through  the  mercury  from  the  centre  to  the  circumference. 
There  are  thus  radial  electric  currents  in  the  disc  of  mercury 
and  a  magnetic  field  perpendicular  to  its  surface.  Under 
these  circumstances,  the  disc  of  mercury  is  set  in  revolution. 
The  movement  of  the  mercury  is  resisted  by  the  friction  of 
it  against  the  sides  of  the  cavity  in  which  it  is  contained. 
The  driving  force  is  thus  nearly  proportional  to  the  square 
of  the  current  strength,  and  the  retarding  force  to  the  square 
of  the  speed  of  revolution.  Hence  it  follows  that  the  number 
of  revolutions  in  a  given  time  is  proportional  to  the  quantity 
of  electricity  which  has  passed  through  the  meter  in  that 
time.  The  revolutions  of  the  mercury  are  counted  by  means 
of  a  delicate  vane  immersed  in  the  mercury,  which  in  turn 
drives  a  counting  mechanism.  The  meter  is  adjusted  so  as 
to  show  on  the  dials  the  number  of  Board  of  Trade  units 
of  electrical  energy  which  have  passed  through  it,  on  the 
assumption  that  it  is  supplied  at  100  volts.  This  meter  is 
very  compact.  It  is  only,  however,  applicable  in  the  case 
of  continuous  currents. 

As  a  good  example  of  an  energy  meter  we  may  select  for 
description  the  watt-hour  meter  of  Elihu  Thomson.  This 
meter  (see  Fig.  98)  is  in  reality  a  small  dynamo,  without 
iron  either  in  its  field  magnets  or  armature.  The  field 
magnets  are  coils  through  which  the  current  to  be  measured 
flows.  The  armature  is  a  coil  fixed  as  a  shunt  across  the 
mains.  Hence  the  magnetic  field  due  to  the  current  in  the 
field  coils  is  proportional  to  that  current.  The  armature 
current  is  proportional  to  the  voltage  of  the  circuit.  Accord- 
ingly the  force  driving  the  armature  is  proportional  to 
the  product  of  the  values  of  the  current  and  the  voltage — 
that  is,  to  the  power  given  to  the  consumer.  The  motion 
of  the  armature  is  retarded  by  placing  on  its  axis  a  copper 
disc,  which  moves  between  fixed  permanent  magnets.  This 


256    ELECTEIG  LAMPS  AND  ELECTEIC  LIGHTING. 

disc  has,  then,  eddy  currents  set  up  in  it,  and  experiences 
a  retarding  force  proportional  to  its  speed.  Accordingly  the 
driving  force  is  proportional  to  the  product  of  the  current 
and  voltage  of  the  main  circuit,  and  the  retarding  force  is 
proportional  to  the  speed.  Hence,  when  the  armature  has 
attained  a  steady  speed,  the  number  of  revolutions  it  makes 


FIG.  98.— Elihu  Thomson  Electric  Meter. 

in  any  given  time  is  proportional  to  the  electrical  energy 
which  has  passed  through  it.  The  armature  shaft  is  attached 
to  a  counting  mechanism,  which  shows  on  dials  the  energy 
in  Board  of  Trade  units  which  have  passed  through  the 
meter.  This  meter  is  capable  of  being  used  either  on  alter- 
nating current  or  continuous  current  circuits. 


ELECTRIC  DISTRIBUTION.  257 

In  addition  to  the  above-described  motor  and  electrolytic 
meters,  there  is  a  large  class  of  meters  called  integrating 
meters.  In  these  appliances  one  portion  of  the  apparatus 
consists  of  an  ampere  meter,  or  electric  power  meter.  The 
remaining  portion  of  the  instrument  consists  of  a  clock 
recording  time,  and  an  integrating  apparatus,  which  practi- 
cally multiplies  together,  at  short  and  regular  intervals,  the 
value  of  the  current  or  power  passing  through  the  meter  and 
the  value  of  the  time  interval,  and  adds  up  the  results, 
presenting  the  total  as  a  time  integral  in  the  form  of  a  record 
of  ampere-hours  or  watt-hours  on  a  series  of  recording  dials. 
These  meters  are  generally  more  complicated  in  construction 
than  the  motor  meters.  A  good  example  of  an  integrating 
meter  is  the  recently  introduced  Johnson  and  Phillips  meter, 
of  which  a  view  is  seen  in  Fig.  99.*  The  meter  consists 
of  an  electrically-driven  clock,  actuating  a  counting  mechanism 
through  a  gear  whose  ratio  is  controlled  by  the  current  or 
power  passing  to  the  lamps.  The  clock  part  is  kept  going  by 
an  ingenious  arrangement  which  imparts  a  little  push  to  the 
pendulum  whenever  the  amplitude  of  its  vibration  falls  below 
a  certain  value.  The  ammeter  part  of  the  instrument  consists 
of  a  coil  of  wire,  through  which  the  house  current  passes. 
Into  this  is  drawn  a  soft  iron  plunger  consisting  of  a  small 
bundle  of  iron  wire,  and  the  magnetic  forces  suck  this  plunger 
into  the  coil  against  the  attractive  forces  of  two  non-magnetic 
springs.  A  needle  in  connection  with  the  plunger  indicates 
on  a  scale  the  current  passing.  The  integrating  part  of  the 
instrument  consists  of  a  revolving  crank  driven  by  the  clock, 
which  makes  two  revolutions  a  minute.  Each  revolution 
depresses  a  lever,  and,  by  a  ratchet  and  pawl  arrangement, 
advances  the  first  wheel  of  the  counting  dials.  The  extent  to 
which  the  pawl  is  advanced  each  revolution  is  governed  by 
the  deflection  of  the  ammeter  needle.  Hence  the  number  of 

*  For  further  details  see  The  Electrician,  August  18,  1899. 

s 


258   ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

revolutions  of  the  counting  dials  in  a  given  time  is  proportional 
to  the  time  and  to  the  deflection  of  the  ammeter  needle — that 


FIG.  99.— Front  View  of  the  Johnson  and  Phillips  Meter, 
is,   to  their  product,  or  to  the  total  quantity  of  electricity 
which  has. passed. 


ELECTEIC  DISTRIBUTION.  259 

Considerable  discussion  has  taken  place  amongst  station 
engineers  as  to  the  relative  advantages  of  quantity  and  energy 
meters.  The  former  have  the  advantage  in  simplicity  of 
construction,  the  latter  in  accuracy  of  reading  under  all 
conditions  of  varying  voltage. 

One  great  advantage  which  electric  meters  possess  over  gas 
meters  is  that  the  former  are  so  easily  checked  for  accuracy 
in  position.  If  a  householder  having  continuous  current  laid 
on  desires  to  check  his  meter  he  should  insert  in  series  with 
the  house  meter  an  accurate  ammeter,  and  across  the 
terminals  of  the  meter  an  accurate  voltmeter.  Then  turn  on 
a  certain  number  of  lamps  in  the  house  for  one  or  more  hours 
and  carefully  note  the  voltmeter  reading  and  the  ammeter 
readings.  If  these  quantities  vary  during  the  time  of 
observation,  the  mean  value  should  be  taken,  the  product 
of  these  readings  gives  the  power  in  watts  given  to  the 
lamps,  and  the  product  of  watts  and  number  of  hours  run, 
divided  by  1,000,  gives  the  energy  in  Board  of  Trade  units 
taken  up.  This  should  agree  with  the  difference  between 
the  house  meter  readings  at  the  beginning  and  end  of  the 
run.  The  same  process  can  be  applied  in  the  case  of  an 
alternating  current  system  provided  it  is  used  only  for 
supplying  current  to  incandescent  lamps.  If  motors  are 
used  on  the  circuit  then  it  would  be  necessary  to  measure 
the  power  taken  up  with  a  properly  designed  alternating 
current  wattmeter. 

Quite  recently  meters  have  been  designed  called  prepayment 
meters,  which  are  set  in  operation  by  inserting  a  coin  into  a 
slot,  and  then  deliver  to  the  consumer  a  fixed  amount  of 
electric  energy  thus  prepaid  for. 

One  of  the  best  known  of  these  prepayment  meters  is  that 
designed  by  Messrs.  Long  and  Schattner.  This  meter  is  an 
electrolytic  meter,  and  consists  of  a  copper  cell  containing 


260    ELECTEIC  LAMPS  AND  ELECTRIC  LIGHTING. 

a  solution  of  sulphate  of  copper,  in  which  hangs  a  copper 
plate  suspended  from  a  lever  A  (see  Figs.  100  and  101).  At 
the  end  of  this  lever  are  two  pins,  Hx  H2,  which  dip  into 
mercury  cups  and  close  the  main  circuit.  The  lever  also  carries 
two  buckets,  K  and  E.  The  lever  is  so  balanced  that  it  just 
tips  up  and  opens  the  circuit.  If  a  coin  (say  one  shilling)  is 
then  placed  in  the  bucket  K,  it  over-balances  the  lever  and 
closes  the  circuit.  The  customer  can  now  light  up  his  lamps. 
The  current  to  these  lamps,  however,  partly  passes  through 
the  solution  of  sulphate  of  copper,  and  takes  copper  off  the 


FIG.  100. — Long-Schattner  Prepayment  Meter. 

suspended  plate.  When  the  weight  of  this  is  reduced  by  the 
weight  of  one  shilling  the  lever  A  tilts  up  and  opens  the 
circuit.  The  adjustments  are  so  made  that  the  shilling  put 
into  the  bucket  just  pays  for  the  electric  energy  which  has 
then  passed.  Another  shilling's  worth  may  then  be  obtained 
and  so  on.  When  the  collector  comes  round  to  examine  the 
meter  and  remove  the  coins,  he  places  in  the  other  bucket 
equivalent  brass  weights  so  that  the  meter  goes  on  measuring 
out  as  before  until  the  copper  plate  is  entirely  dissolved  away. 
The  whole  meter  is  enclosed  in  a  sealed  case  provided  with  a 
slot  and  guiding  tube  for  receiving  the  coins. 


ELECTRIC  DISTRIBUTION. 


261 


In  the  commercial  distribution  and  sale  of  electric  energy 
it  has  been  long  known  that  the  value  of  a  consumer  to  the 
supply  station  does  not  depend  so  much  upon  the  total 
amount  of  energy  taken  as  upon  the  amount  used  per  lamp 
per  annum.  A  consumer  who  has  one  hundred  lamps  in  his 
premises  and  uses  them  on  an  average  one  hour  per  day  is 
not  nearly  so  valuable  to  the  supply  station  as  one  who  has 
twenty  lamps  of  the  same  kind  and  uses  them  on  an  average 
five  hours  a  day.  The  former  customer  may  in  fact  cause  a 
loss  to  the  suppliers.  Hence  of  recent  years  a  system  has 
come  into  use  (first  suggested  and  worked  out  by  Mr.  A. 
Wright,  chief  engineer  of  the  Brighton  Corporation  Electric 


H,  &  H2 
&F2 


FIG.  101. 

Supply  Works),  by  which  the  average  hours  of  use  of  the 
lamps  in  a  consumer's  premises  is  determined  at  the  same 
time  as  his  total  consumption  in  electric  energy  units  is 
measured.  The  charge  per  unit  is  then  made  on  a  sliding 
scale,  so  that  the  units  for  the  first  hour's  use  of  the  lamp 
each  day  are  charged  at  a  higher  rate  than  the  units  for 
subsequent  hours'  use.  The  device  for  marking  this  measure- 
ment is  called  a  Wright's  maximum  demand  meter  (see 
Fig.  102).  It  consists  of  a  bent  (J  tube,  having  at  one  end  a 
bulb  surrounded  by  a  coil  of  wire  through  which  the  customer's 
current  passes.  The  other  end  of  the  tube  has  a  bent-over 
trap  bulb.  The  tube  is  partly  filled  with  mercury.  When 
the  current  passes  through  this  coil  it  heats  it  and  expands 


262    ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

the  air  in  the  enclosed  bulb.  This  forces  some  mercury  over 
into  the  trap  bulb,  and  when  the  current  is  stopped  the 
amount  by  which  the  mercury  in  the  left  hand  leg  of  the  U 


Closed.  Open. 

FIG.  102.— Wright's  Demand  Indicator. 

tube  is  depressed  is  a  measure  of  the  maximum  current  which 
has  passed  into  the  premises.  The  apparatus  is  re-set  by 
tilting  it,  so  as  to  make  the  mercury  fall  back  into  the  tube. 


ELECTRIC  DISTRIBUTION.  263 

The  meter  reader  has,  therefore,  the  power  to  discover  not 
only  the  total  units  taken  by  the  consumer,  but  also  the 
maximum  current  or,  which  is  the  same  thing,  the  maximum 
number  of  lamps  he  has  ever  had  in  use  at  one  time.  Hence 
a  simple  calculation  shows  the  number  of  units  per  lamp, 
and,  therefore,  the  average  hours  of  use  of  each  lamp. 

Thus,  suppose  the  maximum  demand  meter  shows  that  a 
customer  has  taken  a  maximum  current  of  20  amperes,  and 
his  total  units  used  in  three  months  is  600  B.T.U.  Reckoning 
his  lamps,  as  8  c.p.  lamps  and  the  servic'e  pressure  as 
100  volts,  it  is  clear  that  the  maximum  power  he  has  called 
for  in  watts  is  2,000  watts,  but  his  average  power  is  275  watts, 
since  there  are  2,184  hours  in  thirteen  weeks,  and  275  x  2184 
=  600,000.  Hence  his  average  use  of  each  lamp  per  day  is 

'      x  24  =  3-3  hours.     He  has  used  600  units  in  91  days. 

On  the  Wright  system  of  charge  the  customer  is  not  charged 
an  equal  amount  for  all  units  taken.  He  is  charged,  say,  7d. 
for  the  first  two  hours'  use  and  3|d.  for  all  subsequently  used 
units.  Hence,  in  the  above  case,  the  600  units  are  charged 
as  follows :  360  units  at  7d.,  and  240  units  at  3jd.  If  his 
average  use  had  been  only  two  hours  per  day,  it  would  all 
have  been  charged  at  7d.  If  five  hours  per  day  the  average 
cost  per  unit  would  only  be  4-9d.  Hence  that  customer  gets 
an  advantage  who  uses  his  lamps  the  longest  number  of  hours, 
and  not  the  one  who  takes  most  units. 

The  Wright  system  is  now  largely  adopted,  and  has  had  a 
very  beneficial  effect  in  increasing  the  profits  of  the  electric 
supply  business. 

For  additional  details  on  electric  light  fittings,  systems  of 
supply,  and  the  construction  of  electric  lamps,  the  reader 
must  be  referred  to  the  other  manuals  which  deal  with  each 
of  these  portions  of  the  subject  in  a  more  complete  manner 
than  it  has  been  possible  to  do  in  these  short  Lectures, 


INDEX    TO    CONTENTS. 


INDEX. 


PAGE 

A   Glance   Backward   Over   Twenty 

Years  

Absorption  of  Light 31 

Ageing  of  Carbon   Filament  Lamps 

88,  89, 133 

Alternate  Current  Transformer 212 

Alternator 218 

Alternating  Current  Arc 189 

Alternating  Current 161 

Ampere    9 

The  Definition  of    10 

Ampere-meter    69 

Ampere  turns 197 

Amplitude 28 

Annulment  of  Edison  Effect    109 

Arc,  The  Electric  151 

Arc,  Crater  Temperature  of 155 

Arc  Discharge    146 

between  Various  Materials 166 

Arc  Lamps 191 

Arrangement  of 190 

Carbons  179 

Enclosed     185 

Hand  Regulated    149 

Inverted 183 

Mechanism 177 

Arc,  Appearance  of  the  Electric 153 

Armoured  Cable     220 

Artistic  Electric  Lighting    102 

Bamboo  Filament  56 

Battery,  Secondary  244 

Bernstein  Glow  Lamp  58 

Blackening  of  Incandescent  Lamps...  74 

Board  of  Trade  Unit     90 


PAGE 
Board  of   Trade  Electrical    Inquiry 

of  1889    3 

Booster    248 

Bottomley's  Experiments  on  Radia- 
tion    23 

Brightness  33 

Comparison  of   34 

Brush  Discharge    140 

Brussels,  Electric  Lighting  Station  at  241 

Bunsen  Photometer  ; 42 

Cable,  Armoured   220 

Concentric 220 

Candle-Foot    38 

Candle,  Parliamentary,  Standard    ...  38 

Candle-Power    " 69 

Decrease  of,  in  Incan- 
descent Lamps  ...    85,  86 

of  Incandescent  Lamp, 

variation    with  Cur- 
rent   70 

Carbon,  Advantages  of,  for  the  Pro- 
duction of  Electric  Arcs...  166 
Allotropic  Forms  of    52 

Boiling  Point  of   167 

Deposit  of,  in  Glow  Lamps  75 

Filaments 54,55 

• • Properties  of 31 

Ageing   of  133 

Lamps  made  with  126 

Preparation  of    ...  54 
Process  of  produc- 
tion    127 

Treated   130 

Molecule,  Negative  Charge  of  111 


270 


INDEX. 


PAGK 

Horse-power,  Definition  of   .« 20 

Hydraulic  Gradient  12 

Power,     Transmission     of 

from  Tivoli...  228 

Illuminating  Power,  Unit  of   40 

Illumination,  Standard  of    38 

Unit  of    40 

Incandescent  Lamp,  Evolution  of   ...       52 

Invention  of    ...       52 

Incandescent   Electric  Lamps,    Best 
Arrangement  of,  for  Illuminating 

Purposes  98,99 

Incandescent  Lamp 51,  54 

Blackening  of...  74,  75 

Different    Forms 

of  56,57,58 

Incandescent    Lamps,  High- Voltage     124 
Incandescent  Lighting  of  Interiors  100, 101 

Artistic    99 

Problem  of. ..       52 

Intrinsic  Brilliancy  of  Electric  Arc 

Crater 173 

of     Glow    Lamp 

Filament    175 

of  Sun    173 

Invention  of  the  Edison  Glow  Lamp       52 

Inverted  Arc  Lamp  1 83 

Iron  Ring,  Divided,  with  two  Circuits     204 
with  Primary  and  Secon- 
dary Circuits  202 

Joule,  Mr 18 

The  Definition  of  20 

Joule's  Law    18 


Kelvin  Voltmeter  14 

Multicellular  Voltmeter  14 

Kilowatt,  Definition  of  the  21 

Kinetic  Theory  of  Gases  63 

Lamp  Efficiencies , ...  126 

Lane-Fox,  Mr 52 

Life,  Average,  of  Glow  Lamps 135 

Lightning  Protector 157 

Light,  Monochromatic  31 

Reflection  of  31 

Velocity  of 28 

Wave     Lengths    of    Various 

Colours    29 

Lines  of  Force    201 

Load  Diagram    223 

Factor    95 

Low-Pressure  System   234 

Luminosity 33 


Luminosity  of  Various  Coloured  Sur- 
faces       33 

Luminous  Efficiency 26 

of  Glow  Lamps...  83 

of    Incandescent 

Lamps    83 

Magnetic  Circuit    196 

Circuits  with  Air  Gaps .....  207 

Effect  of  Current 195 

Field,  Delineation  of    199 

Field  of  an  Electric  Current  195 

Field  of  Conductor...       195,  196 

Field  of   Magnetised   Iron 

Rings   206 

Field  of  Spiral  Current   ...  196 

Force 203 

Induction  203 

— Reluctivity 20* 

Magneto- motive  Force 203 

Magnetisation  of  Iron  Circuits...    205,  206 

Manufacture  of  Incandescent  Lamps  54 

Marked  Volts 68 

Mean  Free  Path  of  Molecules  63 

Mengarini  Voltmeter ...  80 

Mengarini's  Self-recording  Voltmeter  80 

Mercury  Pump,  Form  of  ...  65 

Metals,  Non-Arcing   156 

Methven  Gas  Standard 47 

Micro-Glow  Lamp 67 

Molecular  Electric  Charge    ....  Ill 

Physics  of  Glow  Lamp 106 

Shadows  in  Glow  Lamp  ...  75 

Moonlight,  Intensity  of     39 

Multicellular  Voltmeter    14 

Multiple-Filament  Glow  Lamp    61 

Nernst  Lamp 136 


Oersted,  Prof.  H.  C 

H.  C.,  his  Discovery  in  1820 


Ohm,  Dr.  G.  S 9,  17 

Ohm's  Law 17 

Parallel  Arc  Lamps   191 

Parchmentised  Thread 55 

Pentane  Standard,  Vernon  Harcourt.  47 

Photometer,  Bunsen 43 

Grease  Spot 44 

Ritchie 42 

Rumford  42 

Photometric  Comparisons 35 

Photometry,  Scientific  Basis  of    37 

of  Glow  Lamps    40 

of     Various  -  Coloured 

Lights 46 


INDEX. 


271 


PAGE 

Plant^  Secondary  Batt  ery    245 

Platinum  Standard  of  Light    47 

Potential,  Electric 11 

Prepayment  Meter    259 

Pressure,  Electric  11 

—  Fall  of,  in  Gas  and  Water 

Pipes 12 

Primary  Circuit 201 

Principles  of  Photometry 36 

Progress  of  Electric  Lighting  in  Ten 

Years 4 

Pump,  Mercury   65 

Purkinje's  Phenomenon    36 

Radiation  at  Different  Temperatures  25 

Luminous       and       Non- 

Luminous .26,27 

Gradual  Production  of,  at 

Different  Temperatures  24 

Reflecting  Power  of  Various  Sources'  104 

Reluctivity 204 

Resistance,  Electric   16 

Retrospect  of  Electric  Lighting  over 

Twenty  Years 1 

Ritchie's  Photometer    40 

Wedge 40 

Rome,  Electric  Lighting  at  225 

Rumford's  Shadow  Photometer 42 

Sawyer  and  Man    62 

Secondary  Battery     244 

Cell  247 

Circuit 205 

Series  Arc  Lamps  190 

or  Parallel  Lighting  191 

Shades  for  Electric  Arc  Lamps    187 

Sir  Humphry  Davy  146 

Sir  W.  H.  Preece   106 

Smashing  Point 86 

Sources  of  Light,  Qualities  of 30 

Spark  Discharge 139 

Sparking  Distance 143 

Pressure 144 

Spectra,  Similar 48 

of  Various  Lights  49 

Spectro-Photometer  49 

Spectrum  of  a  Source  of  Light    49 

Spiral  Electric  Current,  Field  of 196 

Standard,  Candle 38 

ofLight 37 

Stefan's  Law 178 

St.  Pancras  Electric  Lighting  Station  236 
Summary   of   Facts   concerning   the 

Edison  Effect  on  Glow  Lamps 119 

Street  Arc  Lamps 182 

— -—— — — Arrangements  of..  191 


PAGE 

Sun,  Efficiency  of 172 

Intrinsic  Brightness  of 173 

Temperature  of 173 

Sunlight,    Comparison    with    other 

Luminous  Sources  49 

Intensity  of  39 

Table  of  Colours  and  Wave-Lengths 

of  Light   29 

of  Reflecting  Powers  of  Various 

Surfaces    104 

showing  Variation   of  Candle 

Power  with  Voltage  of    In- 
candescent Lamps 90 

Temperatures,  Important 25 

Scale  of 25 

Terms,  Fundamental,  Meaning  of  ...  6 

The  Ampere,  Definition  of    10 

The  Electric  Arc    151 

The  Joule    20 

Three- Wire  Distribution 221,  223 

Three-wire  Electric  Lighting  Station, 

Arrangement  of  Dynamos  in    248 

Tivoli,  Electric  Lighting  Station  at...  227 

Tivoli-Rome  Transmission    226 

Transformer  House 231 

Transformer,  Construction  of  211 

Principle  of 212 

Sub-Centres    220 

Systems    221 

Transmission  of  Electric  Power  210 

Two  Hundred  Volt  Service 124 

Turbines  at  Tivoli 229 

Untreated  Carbon  Filament  Lamps  130 
United  Kingdom,  Electric   Lighting 

in 4 

Units,  Electric    9 

Vacuum  54 

Conductivity  of    118 

Tube  142 

Variation    of    Candle  -  Power    with 

Voltage    71 

Variation    of    Candle  -  Power    with 

Watts  per  Candle-Po wer 73 

Variation     of     Candle-Power     with 

Wattage  in  a  Glow  Lamp 73 

Variation     of     Voltage     of     Public 

Electric  Supply 77 

Various  Sj  stems  of  Electric  Distri- 
bution    219 

Vernon  Harcourt  Pentane  Standard.  47 

Violle  Standard 47 

Volt,  Definition  of  the 16 


272 


INDEX. 


PAGE 

Volta    16 

Voltmeter   14,68 

Holden    78 

Kelvin  Multicellular    14 

Mengarini       80 

Self-recording   80 

Self-recording   (Mengarini)  80 

Watt,  Definition  of  the 20 

Water  Power 209 

Water  Pressure   and    Electric  Pres- 
sure       12  13,210 

Watts  per  Candle-Power 69 


PAGE 

Wave  Lengths   28 

of  Light   29 

Welsbach  Incandescent  Lamp 1 37 

Willans  Engine  218 

Wimshurst  Electric  Machine   13 

Wiring,  Electric 94 

Inspection  of 94 

Work   20 

Wright's  Demand  Indicator 262 

Wright's  System  of  Charge 263 

Wurts'     Discovery    of    Non-Arcing 

Metals 156 

Wurts  Lightning  Protector 157 


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Light  (Electric)  Railway  applications  granted  in  the  preceding  year. 

New  Companies  Registered  and  Companies  Wound  Up  in  the  preceding  year. 

CENTRAL  LIGHTING  STATIONS  OF  THE  UNITED  KINGDOM.—  (Speci-My  -Compiled  Folding  Sheet.) 

COLOURED  SKETCH  MAP  OF  PROVINCIAL  SUPPLY  STATIONS,  with  System  of  Supply,  &c.    (Special. 

A  SHEET  TABLE  GIVING  TECHNICAL  PARTICULARS  OF  THE  ELECTRIC  RAILWAYS  AND  TRAMWAYS 

IN  THE  UNITED  KINGDOM.—  (Specially-Compiled  Folding  Sheet.) 

Exports  of  Electrical  Apparatus  and  Material.        Foreign  Weights  and  Measures,  with  English  Equivalents. 
Colonial  and  Foreign  Import  Duties  on  Electrical  Machinery  and  Apparatus.  —  (Specially  prepared.) 
Table  ot  Systems  of  Charging  for  Electric  Current,  "  Eisy  "  Wiring,  "  Free  "  Lamps,  Motors,  <fcc. 
Wire  Gauge  Tables.    Resistance  and  Weight  Tables.    Comparative  Price  of  Electricity  and  Gas. 
Tables  relating  to  Water  Power.  Hydraulic  Heads,  Rope  Gearing,  &c.        Sinking  Fund  Tables. 
Electric  Traction  Tables  an  -I  Data.    Carrying  Capacity  of  Resistance  Materials. 

Notes  on  Illumination.    Table  and  Curve  of  Hours  of  Lighting.    Post  Office  Regulations.  Electric  Heating  Table 
TELEGRAPHIC  COMMUNICATION  between  Great  Britain,  Colonies,  and  Foreign  Countries. 
TELEGRAPH  TARIFFS  from  the  United  Kingdom  to  all  parts  of  the  World. 
The  Submarine  Cables  of  the  World.    The  Land  Lines  of  the  World.    The  World's  Cable  Fleet, 
Telephone  Statistics,  including  Post  Office  Telephone  Trunk  Line  Regulations,  Forms  of  Telephone  Licence  for 

Local  Authorities  and  Companies,  British  Telephone  Developments,  &c. 
BRITISH  AND  FOREIGN  GOVERNMENTS  AND  THE  CHIEF  OFFICIALS  OF  ELECTRICAL,  ENGINEERING 

AND  WORKS  DEPARTMENTS.-OSpmaZ.)  [Committees.  -(Special.] 

Local  Authorities  and  Chief  Officers  in  London  and  Provinces,  with  Chairmen  of  Lighting  and  Electric  Traction 
Post  Office  Telegraphs  :  Chief  Officials.    Institution  of  Electrical  Engineers  :  Officers,  Rules,  <fec. 
List  of  Universities,  Colleges,  Schools,  &c.  (with  Professors). 
Institution  of  Electrical  Engineers  Benevolent  Fund. 
Socie'te'  Internationale  des  Electriciens,  Verband  Deutscher  Elektrotechnischer,  American  Institute  of  Electrical 

Engineers  and  other  Foreign  Electrical  Societies  :  Officials,  &c. 
Davy-Faraday  Research  Laboratory  of  the  Royal  Institution. 

ELECTRICAL  LIMITED  LIABILITY  COMPANIES,  FINANCIAL  PARTICULARS  CONCERNING.-(,Sp^aZ.) 
DIRECTORY  OF  DIRECTORS  OF  ELECTRICAL  JOINT  STOCK  UNDERTAKINGS  -(Special.) 
The  BIOGRAPHICAL  SECTION  contains  sketches  of  the  careers  of  about  300  of  the  leading  men  in  the  Electrical 

profession,  with  66  Portrait;.  » 

It  will   be  seen  at  once  from  the  above  Abstract   that  no  similar  Publication  so  effectively  caters   for  the 

Electrical  and  Allied  Industries. 


4        "The  Electrician"  Printing  and  Publishing  Co.,  Ltd., 
"THE      ELECTRICIAN7"      SERIES. 

Vol.  I.     SECOND  ISSUE.     466  pages,  price  12s.  6d.,  post  free,  13s. 
Vol   II.     568  pages,  price  12s.  6d.,  post  free  ;  abroad,  13s. 

ELECTROMAGNETIC  THEORY. 

BY  OLIVER  HEAVISIDE. 

EXTRACT     FROM     PREFACE     TO     VOL.    F. 

This  work  is  something  approaching  a  connected  treatise  on  electrical  theory,  though  without 
the  strict  formality  usually  associated  with  a  treatise.  The  following  are  some  of  the  leading 
points  in  this  volume.  The  first  chapter  is  introductory.  The  second  consists  of  an  outline  scheme  of 
the  fundamentals  of  electromagnetic  theory  from  the  Faraday-  Maxwell  point  of  view,  with  some 
small  modifications  and  extensions  upon  Maxwell's  equations.  The  third  chapter  is  devoted  to 
vector  algebra  and  analysis,  in  the  form  used  by  me  in  former  papers.  The  fourth  chapter  is 
devoted  to  the  theory  of  plane  electromagnetic  waves,  and,  being  mainly  descriptive,  may  perhaps  be 
read  with  profit  by  many  who  are  unable  to  tackle  the  mathematical  theory  comprehensively. 
I  have  included  in  the  present  volume  the  application  of  the  theory  (in  duplex  form)  to  straight 
wires,  and  also  an  account  of  the  effects  of  self-induction  and  leakage,  which  are  of  some 
significance  in  present  practice  as  well  as  in  possible  future  developments. 
EXTRACT  FROM  PREFACE  TO  VOL.  II. 

From  one  point  of  view  this  volume  consists  essentially  of  a  detailed  development  of  the 
mathematical  theory  of  the  propagation  of  plane  electromagnetic  waves  in  conducting  dielectrics, 
according  to  Maxwell's  theory,  somewhat  extended.  From  another  point  of  view,  it  is  the 
development  of  the  theory  of  the  propagation  of  waves  along  wires.  But  on  account  of  the 
important  applications,  ranging  from  Atlantic  telegraphy,  through  ordinary  telegraphy  and 
telephony,  to  Hertzian  waves  along  wires,  the  author  has  usually  preferred  to  express  results  in 
terms  of  the  concrete  voltage  and  current,  rather  than  the  specific  electric  and  magnetic  forces 
belonging  to  a  single  tube  of  flux  of  energy.  .  .  .  The  theory  of  the  latest  kind  of  so  called 
wireless  telegraphy  (Lodge.  Marconi,  &c.)  has  been  somewhat  anticipated,  since  the  waves  sent  up 
the  vertical  wire  are  hemispherical,  with  their  equatorial  bases  on  the  ground  or  sea,  which  they 
run  along  in  expanding.  (See  §  60,  Vol.  I.  ;  also  §  393  in  this  volume.)  The  author's  old  predictions 
relating  to  skin  conduction,  and  to  the  possibilities  of  long-distance  telephony  have  been  abundantly 
verified  in  advancing  practice  :  and  his  old  predictions  relating  to  the  behaviour  of  approximately 
distortionless  circuits  have  also  received  fair  support  in  the  quantitative  observation  of  Hertzian 
waves  along  wires.  __ 

SECOND    ISSUE  (19OO).     370  pages,  150  illustrations.    Price  10s.  6d.,  post  free. 

MAGNETIC  INDUCTION  IN  IRON  AND  OTHER  METALS. 

BY  J.  A.  EWING,  M.A.,  B.Sc., 

Pvnfrssnr  nf  Mechanism  and  Applied  Mechanics  in  the  University  of  Cambridge. 

SYNOPSIS    OF    CONTENTS, 

Jitter  an  introductory  chapter,  which  attempts  to  explain  the  fundamental  ideas  and  the 
terminology,  an  account  is  given  of  the  methods  which  are  usually  employed  to  measure  the 
magnetic  quality  of  metals.  Examples  are  then  quoted,  showing  the  results  of  such  measurements 
for  various  specimens  of  iron,  steel,  nickel  and  cobalt.  A  chapter  on  Magnetic  Hysteresis  follows, 
and  then  the  distinctive  feature^  of  induction  by  very  weak  and  by  very  strong  magnetic  forces 
are  separately  described,  with  further  description  of  experimental  methods,  and  with  additional 
numerical  results.  The  influence  of  Temperature  and  the  influence  of  Stress  are  next  discussed. 
The  conception  of  the  Magnetic  Circuit  is  then  explained,  and  some  account  is  given  of  experi- 
ments which  are  best  elucidated  by  making  use  of  this  essentially  modern  method  of  treatment. 
The  book  contains  a  chapter  on  the  Molecular  Theory  of  Magnetic  Induction,  and  the  opportunity 
is  taken  to  refer  to  a  number  of  miscellaneous  experimental  facts  on  which  the  molecular 
theory  has  an  evident  bearing.  The  concluding  chapter  deals  with  the  important  subject  of 
Practical  Magnetic  Testing. 

Strongly  bound  in  Cloth,  price  5s.,  post  free. 

COMPREHENSIVE  INTERNATIONAL  WIRE  TABLES 


OOIDTIDTJOTOIRS- 

BT  W.  S.  BOULT. 

Giving  very  Full  Particulars  of  Wires  and  Cables  ranging  from  2'26in.  to  'OOlin-  diameter,  and 
including  Board  of  Trade,  StaLdard,  Birmingham,  Brown  and  Sharpe,  and  Metric  Gauges  to  the  number  of  469, 
arranged  in  consecutive  oider  according  to  cross-section,  so  that  when  any  conductor  is  sought  the  nearest  sizes 
in  the  other  gauges  can  at  once  be  found.  Any  No.  of  any  one  of  the  gauges  named,  Single  Wire  or  Cable,  can 
be  readily  ascertained.  Full  particulars  are  given  of  Cross-section,  Gauge,  Weight  and  Resistance  in  English, 
American  and  Continental  Units,  one  Table  supplying  the  place  of  the  various  independent  Tables  which 
have  hitherto  been  published.  Great  pains  have  been  taken  throughout  these  Tables  to  ensure  accuracy. 


"The  Electrician"  Printing  and  Publishing  Co.,  Ltd.,        5 

"  THE    ELECTRJCIAN^^JBRIES— continued. 

NOW     READY.-NEW^ANlTlDHEAPER    EDITION  (1900). 

Very  fully  illustrated,  handsomely  bound,  on  good  paper,  price  6s. 

ELECTRIC  LAMPS  AND  ELECTRIC  LIGHTING. 

By  PROF.  J.  A.  FLEMING,  M.A.,  D.Sc.,  F.E.S.,  M.R.I., 

Professor  of  Electrical  Engineering  in  University  College,  London. 

SYNOPSIS    OF    CONTENTS. 

I.— A  Retrospect  over  25  Years— Present  Condition  of  Electric  Lighting— Chief  Properties  of  an  Electric 
Current— Names  of  Electrical  Units— Chemical  Power  of  an  Electric  Current— Hydraulic  Analogies— Electric 
Pressure- Fall  of  Electric  Pressure  Down  a  Conductor— Ohm's  Law— Joule's  Law— Units  of  Work  and  Power— 
The  Watt  as  a  Unit  of  Power— Incandescence  of  a  Platinum  Wire— Spectroscopic  Examination  of  a  Heated 
Wire— Visible  and  Invisible  Radiation— Luminous  Efficiency— Radiation  from  Bodies  at  Various  Temperature,' 
-Efficiency  of  Various  Sources  of  Light — The  Glow  Lamp  and  Arc  Lamp  as  Illuminants — Colours  and  Wave- 
.^engths  of  Rays  of  Light— Similar  and  Dissimilar  Sources  of  Light— Colour-Distinguishing  Power— Causes  o 
Colour— Comparison  of  Brightness  and  Colour— Principles  of  Photometry— Limitations  Due  to  the  Eye- 
Luminosity  and  Candle  Power— Standards  of  Light— Standards  of  Illumination— The  Candle  Foot— Comparison 
of  Sunlight  and  Moonlight— Comparison  of  Lights— Ritchie's  Weda;e  — Rumf ord  and  Bunsen  Photometers- 
Comparison  of  Lights  of  Different  Colours— Speccro  Photometers— Results  of  Investigations. 

II.— The  Evolution  of  the  Incandescent  Lamp— The  Nature  of  the  Problem— Necessary  Conditions— 
Allotropic  Forms  of  Carbon— The  Modern  Glow  Lamp— Processes  for  the  Manufacture  of  the  Filament— Edison- 
Swan  Lamps— The  Expansion  of  Carbon  when  Heatei— Various  Forms  of  Glow  Lamps— Velocity  of  Molecules  of 
Gases— Kinetic  Theory  of  Gases — Processes  for  the  Production  of  High  Vacua — Necessity  for  a  Vacuum — Mean 
Free  Path  of  Gaseous  Molecules -Voltage,  Current  and  Candle  Power  of  Lamps— Watts  per  Candle  Power- 
Characteristic  Curves  of  L  imps — Life  of  Glow  Lamps — Molecular  Shadows — Blackening  of  Glow  Lamps — Salf- 
Recording  Voltmeters— Necessity  for  Constant  Pressure  of  Supply -Changes  Produced  in  Lamps  by  Age- 
Smashing  Point— Efficiency  of  Glow  Lamps— Statistics  of  Age— Variation  of  Candle  Power  with  Varying  Voltage 
—Cost  of  Incandescent  Lighting— Useful  Life  of  Lamps— Importance  of  Careful  "  Wiring  "—Average  Energy 
Consumption  of  Lamps  in  Various  Places — Load  Factors — Methods  of  Glow  Lamp  Illumination  for  Production 
of  Best  Effects— Artistic  Electric  Lighting— Molecular  Physics  of  the  Glow  Lamp— High  Voltage  Lamps  — 
Varieties  of  Carbon— Densities  and  Resistances— Deterioration  of  Carbon  Lamps— High  Efficiency  La-nps— 
Recently  Suggested  Improvements. 

III.— Forms  of  Electric  Discharge— Vacuum  Tubes-Sparking  Distance— The  Electric  Arc -The  Optical 
Projection  of  the  Arc— The  Arc  a  Flexible  Conductor— The  High  Temperature  of  the  Arc— Non- Arcing  Metals- 
Lightning  Protectors— The  Distribution  of  Light  from  the  Arc— Continuous  and  Alternating  Current  Arcs— 
Voltage  Required  to  Produce  an  Arc— The  Physical  Actions  in  the  Arc— The  Changes  in  the  Carbons— The 
Distribution  of  a  Potential  in  the  Arc— The  Unilateral  Conductivity  of  the  Arc— The  Temperature  of  the  Crater 
—Comparison  with  Solar  Temperature— The  "Watfs  per  Candle"  of  the  Sun— Intrinsic  Brightness  and  Dissi- 
patiye  Power  of  Heated  Surfaces— Comparison  of  Glow  Lamp,  Arc  Lamp  and  Sun  in  Respect  of  Brightness  and 
Radiation— Arc  Lamp  Mechanism— Arc  Lamp  Carbons— The  Hissing  of  Arc  Lamps— The  Applications  of  Arc 
Lamps— Inverted  Arcs— Series  and  Parallel  Arc  Lighting— The  Enclosed  Arc  Lamp. 

IV.— The  Generation  and  Distribution  of  Electric  Current -The  Magnetic  Action  of  an  Electric  Current— 
The  Magnetic  Field  of  a  Spiral  Current— The  Induction  of  Electric  Currents— The  Peculiar  Magnetic  Property 
of  Iron— Iron  and  Air  Magnetic  Circuits -The  Typical  Cases  of  an  Iron  Conduit  With  and  Without  Air  Gaps  — 
The  Prototypical  Forms  of  Dynamo  and  Transformer— The  Transformation  of  Electrical  Energy— Hydraulic 
Illustrations— The  Mechanical  Analogue  of  a  Transformer— The  M  -de  of  Construction  of  an  Alternate  Current 
Transformer— The  Fundamental  Principle  of  all  Dynamo  Electric  Machines— Alternating  aid  Direct  Current 
Dynamos— Alternating  Current  Systems  of  Electrical  Distribution -Description  of  the  Electric  Lighting  Station 
in  Rome— The  Tivoli-Rome  Electric  Transmission— Views  of  the  Tivoli  Station— Continuous  Current  Systems— 
The  Three- Wire  System— Description  of  St.  Pancras  Vestry  Electric  Lighting  Station— Liverpool,  Glasgow  and 
Brussels  Electric  Lighting  Stations— Direct  Driven  Dynamos -Alternating  and  Continuous  Current  Systems 
Contrasted— Secondary  Batteries— House  and  Maximum  Demand  Meteis. 

NEW    EDITION    READY    EARLY    IN    1900. 

800  pages,  specially  bound  bookwise  to  lie  open,  7s.  6d.  nett,  post  free  ?S.  9d.  (abroad,  8s.,  post  free). 

A.   POCKET  -  BOOK!   OF 

ELECTRICAL  ENGINEERING  FORMULA  &c. 

BY  W.  GEIPEL  AND   H.  KILGOUR. 

With  the  extension  of  all  branches  of  Electrical  Engineering  (and  particularly  the  heavier 
branches),  the  need  of  a  publication  of  the  Pocket-Bouk  style  dealing  practically  therewith 
increases  ;  for  while  there  are  many  such  books  referring  to  Mechanical  Engineering,  and  several 
dealing  almost  exclusively  with  the  lighter  branches  of  electrical  work,  noue  of  these  suffice  for  the 
purposes  of  the  numerous  body  of  Electrical  Engineers  engaged  in  the  application  of  electricity  to 
Lighting,  _  Traction,  Transmission  of  Power,  Metallurgy,  and  Chemical  Manufacturing:.  It  is  to 
supply  this  real  want  that  this  most  comprehensive  book  has  been  prepared. 

Compiled  to  some  extent  on  the  lines  of  other  pocket-books,  the  rules  and  formulae  in  general 
use  among  Electricians  and  Electrical  Engineers  all  over  the  world  have  been  supplemented  by 
brief  and,  it  is  hoped,  clear  descriptions  of  the  various  subjects  treated,  as  well  as  by  concise 
articles  and  hints  on  the  construction  and  management  of  various  plant  and  machinery. 

No  pains  have  been  spared  in  compiling  the  various  sections  to  bring  the  book  thoroughly  up 
to  date  ;  and  while  much  original  matter  is  given,  that  which  is  not  original  has  been  carefully 
selected,  and,  where  necessary,  corrected.  Where  authorities  differ,  as  far  as  practicable  a  mean  has 
been  taken,  the  differing  formulae  being  quoted  for  guidance. 

Specimen  Pages,  <kc.,  sent  post  free  on  apjlicalion. 


6        "The  Electrician"  Printing  and  Publishing  Co.,  Ltd., 

NEW  EDITION— Almost  entirely  Rewritten,  and  brought  up  to  date, 

More  than  600 pages  and  213  Illustrations,  12s.  6d.  post  free;  abroad,  13s. 

THE  ALTERNATE  CURRENT  TRANSFORMER 

IN   THEORY  AND   PRACTICE. 

By  J.   A.   FLEMING,   M.A.,   D.Sc.,   F.E.S.,  M.R.I.,   &c., 

Professor  of  Electrical  Engineering  at  University  College,  London. 

Since  the  first  edition  of  this  Treatise  was  published,  the  study  of  the  properties  and  applications  of 
alternating  electric  currents  has  made  enormous  progress.  .  .  .  The  author  has,  accordingly,  rewritten  the 
greater  part  of  the  chapters,  and  availed  himself  of  various  criticisms,  with  the  desire  of  removing  mistakes  and 
remedying  defects  of  treatment.  In  the  hope  that  this  will  be  found  to  render  the  book  still  useful  to  the  increas- 
ing numbers  of  those  who  are  practically  engaged  in  alternating-current  work,  he  has  sought,  as  far  as  possible, 
to  avoid :  academic  methods  and  keep  in  touch  with  the  necessities  of  the  student  who  has  to  deal  with  the 
subject  not  as  a  basis  for  mathematical  gymnastics  but  with  the  object  of  acquiring  practically  useful  knowledge. 

SYNOPSIS  OF  CONTENTS  OF  VOL,  !, 

CHAPTER  I.— Introductory.— Faraday's  Electrical  Researches— Faraday's  Theories— Magneto-Electric 
Induction— Views  of  Maxwell,  Helmholtz  and  Kelvin— Action  at  a  Distance -The  Electro-Magnetic  Medium— 
i^nseph  Henry's  investigations. 

CHAPTER  II.— Electro-Magnetic  Induction.— Magnetic  Force  and  Magnetic  Fields— MagneticJForce 
<.mr  Conductors— Typical  Cases— Magneto-Motive  Force  and  Magnetic  Induction— Flux  of  Force— Magnetic 
1  enneability—  Lin es  of  Induction— Faraday's  Law  of  Induction — Electromotive  Force  Due  to  the  Change  of  Induc- 
tion—The Magnetic  Circuit— Magnetic  Resistance— Lines  of  Induction  of  Closed  and  Open  Magnetic  Circuits- 
Fundamental  Relations  Between  Magnetic  Force  and  Magnetic  Induction — Intensity  of  Magnetic  ion — Magnetic 
Moment-Lines  and  Tubes  of  Magnetic  Induction— Curves  of  Magnetisation— Permeability  curves— Determina- 
tion of  Permeability— Magnetic  Hysteresis— Hysteresis  Curves— H.B.  Diagram— Effect  of  Temperature  on  Hyster- 
esis—Electromotive Force  of  Induction— Graphical  Representations— Electromotive  Force  of  Self-Induction. 

CHAPTER  III.— Tne  Theory  of  Simple  Periodic  Currents.— Variable  and  Steady  Flow— Current 
and  Electromotive  Force  Curves -Fourier's  Theorem— Mechanical  Harmonographs— Mathematical  Sketch  of 
Fourier's  Theorem— Practical  Application  of  Fourier's  Theorem  to  the  Harmonic  Analysis  of  a  Periodic  Curve 
Simple  Periodic  Currents  and  Electromotive  Forces— Description  of  a  Simple  Periodic  Curve— The  Mean  Value 
of  the  Ordmate  of  a  Sine  Curve — The  Square  Root  of  the  Mean  of  the  Squares  of  the  Ordinates  of  a  Simple 
Periodic  Curve— Derived  Curves— Inductance  and  Inductive  Circuits;  Inductance,  Resistance  and  Capacity  of 
Circuits  ;  Inductive  and  Non-inductive  Circuits— Faraday's  and  Henry's  Experiments  on  Self-induction  -  Edlund's 
and  Maxwell's  Arrangement  for  Exhibiting  Inductance  of  Circuits -Electro- Magnetic  Momentum— Electrotonic 
State  and  Electromagnetic  Energy— Co-efficient  of  Mutual  Induction;  Energy  of  Two  Circuits— The  Unit  of 
Inductance— Value  of  the  Self-induction  in  Henry's  for  Various  Instruments— Current  Growth  in  Inductive 
Circuits— Analogy  of  Current  and  Velocity  Change ;  Fundamental  Equations  for  Current  Growth  in  Inductive 
rircuits— Equation  for  the  Establishment  of  a  Steady  Current  in  Inductive  Circuits— Time  Constant  of  an 
Inductive  Circuit-  Logarithmic  Curves— Instantaneous  Value  of  Simple  Periodic  Current— Solution  of  Current 
Equation- Impedance  of  Inductive  Circuit— Relations  of  Impressed  Electromotive  Force,  Current  and  Im- 
pedance- GeometMcal  Illustrations— Impressed  and  Effecive  Electromotive  Forces— Clock  Diagram  of  Electro- 
motive Forces  in  Inductive  Circuit — Triangle  Representing  Resistance,  Impedance  and  Reactance"  of  Ci  cuit— 
'Jhe  Mean  Value  of  the  Power  of  a  Periodic  Current— Geometrical  Theorem --Power  Curves  for  Inductive  and 
Non-inductive  Circuits— Experimental  Measurement  of  Periodic  Currents  and  Electromotive  Forces — Mean 
Square  Value— Method  of  Measuring  the  True  Mean  Power  Given  to  an  Inductive  Circuit— Theory  of  the 
Wattmeter— Divided  Circuits— Important  Trigonometrical  Lemma— Impedance  of  Branched  Circuits— Watt- 
meter Measurement  of  Periodic  Power— Mutual  Induction  of  Two  Circuits  of  Constant  Induct  «nc3—  The 
Flow  of  Simple  Periodic  Currents  into  a  Condenser— Time  Constant  of  a  Condenser— Charging  a  Condenser 
through  a  Resistance— Condenser  equation— Annulmint  of  Inductance  by  Capacity— Representation  of  Periodic 
Currents  by  Polar  Diagrams -Initial  Conditions  on  Starting  Current  Flow  in  Inductive  Circuits— Complex 
Periodic  Functions— Apparent  and  True  Power  given  to  Inductive  Circuits— Power  Factor. 

CHAPTER  IV.-  Mutual  and  Self-Induction.— The  Researches  of  Joseph  Henry— Experiments  with 
Coils  and  Bobbins  ;  Discovery  of  Self-induction— Mutual  Induction— Induction  at  a  Distance— Induction  between 
Telephone  Circuits— Induction  over  Great  Distances— Induced  Currents  of  Higher  Orders— Inductive  Effects  by 
Transient  Electric  Currents— Magnetic  Screening— Direction  of  Induced  Currents— Various  Qualities  of  an 
Induced  Current— Elementary  Theory  of  Mutual  Induction  of  Two  Circuits— Theory  of  Induction  Coil  with  Non- 
Magnetic  Core— Comparison  of  Theory  and  Experiment— Duration  of  Induced  Currents— Magnetic  Screening 
Action  of  Good-Conducting  Masses— Faraday's  and  Henry's  Experiments— Willoughby  Smith's  Investigations  on 
Magnetic  Screening— Dove  s  Experiments  and  Henry's  Views  on  same— The  Reaction  of  the  Secondary  Currents 
on  toe  Primary  Circuit  in  the  Case  of  an  Induction  Coil— Induction  Balance  and  Sonometer— Transmission  of 
Alternating  Currents  through  Conductors — Prof.  Hughes'  Experiments — Lord  Rayleigh's  Researches — Flow  of 
Current  through  Conductor- Surface  Flow  of  Alternating  Currents— Increased  Resistance  of  Conductors  for 
Alternating  Currents  of  High  Frequency— Limiting  Size  of  Conductors  for  Conveyance  of  Alternating  Currents  - 
Stepban's  Analogies— Electro-Magnetic  Repulsion— Elihu  Thomson's  Experiments— Electro-Magnetic  Rotations. 
CHAPTER  V.-Dynamical  Theory  of  Induction.— Electro-Magnetic  Theory— Faraday's  Concep- 
tion of  an  Electro-Magnet'c  Medium— Maxwell's  Suggestion— The  Luminiferous  Ether— Maxwells  Theory  of 
Electric  Displacement— Electric  Elasticity  of  the  Medium— Displacement  and  Conduction  Currents— Electro- 
motive Intensity— Displacement  Currents  and  Displacement  Waves— Theory  of  Molecular  Vortices— Mechanical 
Analogy— Comparison  of  Theory  and  Experiment— Maxwell's  Law  Connecting  Dielectric  Constant  and  Refrac- 
tivity  of  a  Dielectric— Tables  of  Comparison— Velocity  of  Propagation  of  Electro-Magnetic  Disturbances- 
Values  of  "<v"— Vector  Potential—  Electrical  Oscillation— Charge  and  Discharge  of  Leyden  Jar— The  Function 
of  th«  Condenser  in  an  Induction  Coil— Impulsive  Discharges  and  Relation  of  Inductance  Thereto— Impulsive 
Impedance— Hertz's  Researches— Experimental  Determination  of  the  Velocity  of  Electro-Magnetic  Waves. 

CHAPTER  VJ.-The  Induction  Coil  and  Transformer.- General  Description  of  the  Action  of 
the  Transfoimer  or  Induction  Coil— The  Delineation  of  Periodic  Curves  of  Current  and  Electromotive  Force- 
Curve  Tracers — Transformer  Diagrams  — Curves  of  Electromotive  Force— Cuirent  and  Induction  in  Cases  of 
Various  Transformers  Taken  off  Various  Alternators— Open -Circuit  Current  of  Transformers— Symmetry  of 
Transformer  Curves— Harmonic  Analysis  of  Transformer  Curves— Power  and  Hysteresis  Curves— Hysteresis 
Curves  of  Various  Transformers— Efficiency  of  Transformers— Efficiency  Curves  of  Transformers— Tables  of 
Efficiencies— Current  Diagrams  of  a  Transformer— Tables  of  Complete  Tests— The  Power  Factor— Open  and 
Closed  Circuit  Transformers— Magnetic  Leakage  and  Secondary  Drop— Various  Causes  of  Secondary  Drop- 
Determination  of  Magetic  Leakage— Investigations  cf  the  Author  and  Dr.  Roesslei— Form  Factor  and  Amplitude 
Factor  of  a  Periodic  Curve— General  Analytical  Theory  of  the  Transformer  and  Induction  Coil. 


"The  Electrician"  Printing  and  Publishing  Co.,  Ltd.,      _7 
"THE  ELECTRICIAN"   SERIES— contimi**. 


THIRD    ISSUE.       More  than  600 pages  and  over  300  illustrations.     Price  12s.  6d.,  post  fret  ; 

abroad,  13s. 

THE  ALTERNATE  CURRENT  TRANSFORMER 

IN   THEORY   AND    PRACTICE. 

By  J.   A.  FLEMING,   M.A.,   D.Sa,  F.E.S.,   M.R.I.,   &c., 

Professor  of  Electrical  Engineering  in  University  College  London. 


SYNOPSIS   OF   CONTENTS   OF   VOL.  2. 
CHAP.  I. — Historical  Development  of  Induction  Coil  and  Transformer. 

The  Evolution  of  the  Induction  Coil — Page's  Researches — Callan's  Induction  Apparatus — 
Sturgeon's  Induction  Coil — Bachhoffner's  Researches — Callan's  Further  Researches — Callan'g 
Great  Induction  Coil — Page's  Induction  Coil— Abbot's  Coil  -  Automatic  Contact  Breakers — 
Ruhmkorff's  Coils  —  Poggendorff's  Experiments — Stohrer's,  Hoarder's,  Ritcbie's  Induction 
Apparatus — Grove's  Experiments — Apps'  Large  Induction  Coils — Jablochkoff's  Patent — Fuller's 
Transformer — Early  Pioneers — Gaulard  and  Gibb? — Zipen^vsky's  Transformers — Improvements 
of  Rankin  Kennedy,  Hopkinson,  Ferranti,  and  others — The  Modem  Transformer  since  1885. 

CHAP.  II.— Distribution  of  Electrical  Energy  by  Transformers. 

Detailed  Descriptions  of  Large  Alternate-Current  Electric  Stations  using  Transformers  in 
Italy,  England,  and  United  States — Descriptions  of  the  Systems  of  Zipernowsky-Deri-Blathy, 
Westinghouse,  Thomson-Houston,  Mordey,  Lowrie  Hall,  Ferranti.  and  others -Plans,  Sections, 
and  Details  of  Central  Stations  using  Transformers — Illustrations  of  Alternators  and  Transformers 
in  Practical  Use  in  all  the  chief  British,  Continental,  and  American  Transformer  Stations. 

CHAP.  III. — Alternate-Current  Electric  Stations. 

General  Design  of  Alternating-Current  Stations,  Engines,  Dynamos,  Boilers — Proper  Choice 
of  Units— Water  Power—Parallel  Working  of  Alternators— Underground  Conductors— Various 
Systems— Concentric  Cables— Capacity  Effects  dependent  on  Use  of  Concentric  Cables — Phenomena 
of  Ferranti  Tubular  Mains — Safety  Devices — Regulation  of  Pressure — Choice  of  Frequency — 
Methods  of  Transformer  Distribution — Sub-Stations — Automatic  Switches. 

CHAP.  IV. — The  Construction  and  Action  of  Transformers. 

Transformer  Indicator  Diagrams— Ryan's  Curves — Curves  of  Current— Electromotive  Force 
and  Induction-Analysis  of  Transformer  Diagrams — Predetermination  of  Eddy  Current  and 
Hysteresis  Loss  in  Iron  Cores — Calculation  and  Design  of  Transformers — Practical  Predetermina- 
tion of  Constants — Practical  Construction  of  Transformers— Experimental  Tests  of  Transformers 
— Measurement  of  Efficiency  of  Transformers— Calometric  Dynamometer  and  Wattmeter  Methods 
— Reduction  of  Results. 

CHAP.  V. — Further  Practical  Application  of  Transformers. 

Electrical  Welding  and  Heating  Transformers  for  producing  Large  Currents  of  Low  Electro- 
motive Force — Theory  of  Electric  Welding— Other  Practical  Applications — Conclusion. 

NEW    EDITION.- JUST    PUBLISHED. 

Fully  Illustrated.      Price  6s.  net,  post  free  ;  abroad  6s.  3d. 
STUDENTS'    GUIDE    TO 

SUBMARINE   CABLE   TESTING. 

By  H.  K.  C.  FISHER  and  J.  C.  H.  DAEBY. 

The  authors  of  this  book  have,  for  some  years  pa&t,  been  engaged  in  the  practical  work  of  Submarine  Cable 
Testing  in  the  Eastern  Extension  Telegraph  Company's  service,  and  have  embodied  their  experience  in  a  Guide 
for  the  use  of  those  in  the  Telegraph  vService  who  desire  to  qualify  themselves  for  the  examinations  which  the 
Cable  Companies  have  recently  instituted.  To  those  desirous  of  entering  the  Cable  Service,  Messrs.  Fisher  and 
Darby's  book  is  indispensable,  as  it  is  now  necessary  for  probationers  to  pass  these  examinations  as  part  of  the 
qualification  for  service. 

1,  2>  and  3,  Salisbury  Court,  Fleet  Street,  London,  F.C. 


8        "The  Electrician"  Printing  and  Publishing  Co.,  Ltd., 
"THE  ELECTRICIAN"  SERIES— continued. 


In  Two  Volumes,  2s.  6d.  each,  post  free  2t.  9d.  each.    Single  Primers,  3d.,  post  free  3\d. 

"THE  ELECTRICIAN"  PRIMERS. 

(FULLY  ILLUSTRATED.) 

A  SCMGS  of  Helpful  Primers  on  Electrical  Subjects  for  the  use  of  those  seeking  a 
Knowledge  of  Electricity— Theoretical  and  Practical. 

CONTENTS. 


Yolume  I.— THEORY. 

Primer  

No. 

1.  The  Effects  of  an  Electric 

Current. 

2.  Conductor  sand  Insulators. 

3.  Ohm's  Law. 

4.  Primary  Batteries. 

6.  Arrangement  of  Batteries. 

6.  Electrolysis. 

7.  Secondary  Batteries. 

8.  Lines  of  Force. 

9.  Magnets. 

10.  Electrical  Units. 

11.  The  Galvanometer. 

12.  Electrical  Measuring  In- 

struments. 

13.  The  Wheatstone  Bridge. 

14.  The  Electrometer. 

15.  The  Induction  CoiL 

16.  Alternating  Currents. 

17.  The  Leyden  Jar. 

18.  Influence  Machines. 
19^Lightning  Protectors. 
20.  Thermopiles. 


The  object  of  "The  Electrician  ' 
Primers  is  to  briefly  describe  in  sim- 
ple and  correct  language  the  present 
state  of  electrical  knowledge.  Each 
Primer  is  short  and  complete  in  itself, 
and  is  devoted  to  the  elucidation  of 
some  special  point  or  the  description 
of  some  special  application.  Theo- 
retical discussion  is  as  far  as  possible 
avoided,  the  principal  facts  being 
stated  and  made  clear  by  reference 
to  the  uses  to  which  they  have  been 
put.  Both  volumes  are  suited  to 
readers  having  little  previous  ac- 
quaintance with  the  subject.  The 
matter  is  brought  up  to  date,  and 
the  illustrations  refer  to  instruments 
and  machinery  in  actual  use  at  the 
present  time.  It  is  hoped  that  the 
Primers  will  be  found  of  use  where- 
ever  the  want  of  a  somewhat 
popularly  written  work  on  electricity 
and  its  industrial  applications,  pub- 
lished at  a  popular  price,  has  been 
felt.  Electricity  Committees  of 
Town  Councils  will  find  the  Primers 
of  great  service.  Artisans  will  ftnd 
the  Primers  useful  in  enabling 
them  to  obtain  clear  notions  of  the 
essential  principles  underlying  the 
apparatus  of  which  they  may  be 
called  upon  to  take  charge. 


Yolumell.— PRACTICE. 

Primer  

21.  The  Electric  Telegraph. 

22.  Automatic    and     Duplex 

Telegraphy. 

23.  The  Laying  and  Repair  of 

Submarine  Cables. 

24.  Testing  Submarine  Cables. 

25.  The  Telephone. 

26.  Dynamos. 

27.  Motors. 

28.  Transformers. 

29.  The  Arc  Lamp. 

30.  The  Incandescent  Lamp. 

31.  Underground  Mains. 

32.  Electric  Meters. 

33.  Electric  Light  Safety  De- 

vices. 

34.  Systems  of  Electric  Distri- 

bution. 

35.  Electric   Transmission   of 

Energy. 

36.  Electric  Traction. 

37.  Electro-Deposition. 

38.  Electric  Welding. 


New  Edition  in  March,  price  3s.,  post  free. 
A  DIGEST  OF  THE 

LAW  OF  ELECTRIC  LIGHTING,  ELECTRIC  TRACTION, 
AND  OTHER  SUBJECTS. 

By  A.  C.  CURTIS-HAYWAKD,  B.A.,  M.T.E.E. 

Being  a  full  critical  abstract  of  the  Electric  Lighting  Acts,  1882  and  1889,  of  the  Tramways  Act,  1870,  and  of 
the  various  documents  emanating  from  the  Hoard  of  Trade  dealing  with  Electric  Lighting  and  Electric  Traction, 
Including  the  Rules  as  to  the  procedure  in  connection  with  applications  to  the  Light  Railway  Commissioners 
lor  Orders  under  the  Light  Railways  Act,  1896.  The  Digest  treats  first  of  the  manner  in  which  persons  desirous 
of  supplying  electricity  must  set  to  work,  and  then  of  their  rights  and  obligations  after  obtaining  Parliamentary 
powers  ;  and  gives  in  a  succinct  form  information  of  great  value  to  Local  Authorities,  Electric  Light  Contractors, 
&c.,  up  to  date.  The  Board  of  Trade  Regulations  as  to  the  Supply  of  Electrical  Energy,  the  London  County 
Council  Regulations  as  to  Overhead  Wires,  Theatre  Lighting,  &c.,  together  with  the  Bye-laws  enforced  in 
pursuance  of  Part  II.  of  the  Public  Health  Acts  Amendment  Act,  1890,  by  the  various  Urban  Sanitary 
Authorities  are  also  given. 

440  PP-     Fourth  Edition.     Price  7s.  6d. 

CORRECTION    OF    ERRORS   IN    CODE   AND   OTHER  TELEGRAMS. 

Containing  about  70,000  example?,  many  of  which  are  the  outcome  of  actual  experience.     A 
large  proportion  of  the  presumed  errors  in  telegrams  may  be  explained  by  a  reference  to  these 
ages.     Invaluable  to  Telegraph  and  Cable   Companies,  and   to  Commercial  Houses  using  Code 
ocabuJarie'". 

I,  2  and  3,  Salisbury  Court,  Fieet  Street,  London,  E.G. 


"The  Electrician"  Printing  and  Publishing  Co.,  Ltd., 
"THE  ELECTRICIAN"  SERIES—  continued. 


Price  12s.  6d.,  post  free;  abroad.  13s. 

MOTIVE    POWER   AND   GEARING 


FOR 

A  Treatise  on  the  Theory  and  Practice  of  the  Mechanical  Equipment  of  Power  Stations  for  Electric 

Supply,  and  for  Electric  Traction. 

BY  E.   TREMLETT   CARTER,   C.E.,   M.I.E.E.,   F.R.A.S.,   F.RS.  (Lond.),  &c. 

650  pages,  200  Illustrations,  Scale  Drawings  and  Folding  Plates,  and  over  80  Tables  of  Engineering  Data. 

IN   ONE   VOLUME. 

Part      I.—  Introductory.  Part  II.—  The  Steam  Engine.  Part  III.—  Gas  and  Oil  Engines. 

Part  IV.—  Water  Power  Plant.  Part  V.—  Gearing.  Part  VI.—  Types  of  Power  Stations 

This  work  presents  to  consulting  engineers,  contractors,  central-station  engineers,  and 
engineering  students  the  latest  and  most  approved  practice  in  the  equipment  and  working  of 
mechanical  plaut  in  electric-power  generating  stations.  Every  part  of  the  work  has  been  brought 
completely  up  to  date  ;  and  especially  in  the  matter  of  the  costs  of  equipment  and  working  the 
latest  available  information  has  been  given.  The  treatise  deals  with  Steam,  Gas,  Oil  and  Hydraulic 
Plant  and  Gearing  ;  and  it  deals  with  these  severally  from  the  three  standpoints  of  (1)  Theory, 
(2)  Practice  and  (3)  Costs 

"MOTIVE  POWER  AND  GEARING  FOR  ELECTRICAL  MACHINERY"  is  a  handbook  of  modern 
electrical  engineering  practice  in  all  par,  f  the  world.  It  offers  to  the  reader  a  means  of 
•comparing  the  central-station  practice  of  the  (Jnited  Kingdom  with  that  of  America,  the  Colonies 
or  other  places  abroad  ;  and  it  enables  him  to  study  the  scientific,  economic  and  financial  principles 
upon  which  the  relative  suitability  of  various  forms  of  practice  is  based,  and  to  apply  these 
.principles  to  the  design  or  working  of  plant  for  any  given  kind  of  work,  whether  for  electrical 
supply  or  for  electric  traction.  It  is  a  treatise  which  should  be  in  the  hands  of  every  electrical 
engineer  throughout  the  world,  as  it  constitutes  the  only  existing  treatise  on  the  Economics  of 
Motive  Power  and  Gearing  for  Electrical  Machinery. 

NEW    EDITION  (1900). 

Over  400  pages,  nearly  250  illustrations.    Price  10s.  6d.,  post  free  ;  abroad,  lls» 

ELECTRIC  MOTIVE   POWER, 

By  ALBION  T.  SNELL,  Assoo.M.lNST.C.E.,  M.I.E.E. 

The  rapid  spread  of  electrical  work  in  collieries,  mines,  and  elsewhere  has  created  a  demand  for  a  practical 
book  on  the  subject  of  transmission  of  power.  Though  much  had  been  written,  there  was  no  single  work  dealing 
•with  the  question  in  a  sufficiently  comprehensive  and  yet  practical  manner  to  be  of  real  use  to  the  mechanical 
or  mining  engineer;  either  the  treatment  was  adapted  for  specialists,  or  it  was  fragmentary,  and  power  work 
was  regarded  as  subservient  to  the  question  of  lighting.  The  Author  has  felt  the  want  of  such  a  book  in  dealing 
with  his  clients  and  others,  and  in  "  ELECTRIC  MOTIVE  POWER  "  has  endeavoured  to  supply  it. 

In  the  introduction  the  limiting  conditions  and  essentials  of  a  power  plant  are  analysed,  and  in  the 
subsequent  chapters  the  power  plant  is  treated  synthetically.  The  dynamo,  motor,  line,  and  details  are 
discussed  both  as  to  function  and  design.  The  various  systems  of  transmitting  and  distributing  power  by  con- 
tinuous and  alternate  currents  are  fully  enlarged  upon,  and  much  practical  informatio  gathered  from  actual 
experience,  is  distributed  under  the  various  divisions.  The  last  two  chapters  deal  exhaustively  with  the 
applications  of  electricity  to  mining  work  in  Great  Britain,  the  Continent  .and  America,  particularly  with 
reference  to  collieries  and  coal-getting,  and  the  results  of  the  extensive  experience  gained  in  this  field  are 
«mbodied. 

In  general,  the  Author's  aim  has  been  to  give  a  sound  digest  of  the  theory  and  practice  of  the  electrical 
transmission  of  power,  which  will  be  of  real  use  to  the  practical  engineer,  and  to  avoid  controversial  .points 
which  lie  in  the  province  of  the  specialist,  and  elementary  proofs  which  properly  belong  to  text-books  on 
electricity  and  magnetism. 

To  meet  the  convenience  of  Continental  readers  and  others,  the  Author  has  prepared 
in  tabular  form  and  in  parallel  columns  the  working  equations  used  in  this  work  in  inch- 
pound-minute  and  centimetre-gramme-second  units,  so  that  they  may  be  readily  used  1» 
either  system. 

1,  2    and  3,  Salisbury  Court,  Fleet  Street,  London,  E.C 


10      "The  Electrician"  Printing  and  Publishing  Co.,  Ltd., 
"THE   ELECTRICIAN"   SERIES— continued. 


Price  5s.,  post  free  ;  abroad,  5s.  6d.     180  pages  and  over  100  illustrations. 

THE  LOCALISATION  §  FAULTS  IN  ELECTRIC  LIGHT  MAINS. 

By  F.  CHARLES  RAPHAEL. 

Although  the  localisation  of  faults  in  telegraph  cables  has  been  dealt  with  fully  in  several 
hand-books  and  pocket-books,  the  treatment  of  faulty  electric  light  and  power  cables  has  never  been 
discussed  in  an  equally  comprehensive  manner.  Beyond  a  few  short  articles  which  have  appeared 
in  the  technical  journals  from  time  to  time,  nothing  has  been  written  on  the  subject.  The  condi- 
tions of  the  problems  presented  for  solution  are,  however,  very  different  in  the  two  cases  ;  faults  in 
telegraph  cables  are  seldom  localised  before  their  resistance  has  become  low  compared  with  the 
resistance  of  the  cable  itself,  while  in  electric  light  work  the  contrary  almost  always  obtains.  This 
fact  alone  entirely  changes  the  method  of  treatment  required  in  the  latter  case,  and  it  has  been  the 
author's  endeavour,  by  dealing  with  the  matter  systematically,  and  as  a  separate  subject,  to 
adequately  fill  a  gap  which  has  hitherto  existed  in  technical  literature. 

The  various  methods  of  insulation  testing  during  working  have  been  collected  and  discussed,  as 
these  tests  may  be  considered  to  belong  to  the  subject. 

Price  6S.,  post  free  ;  abroad  6s.  6<J, 

THE  POTENTIOMETER  AND  ITS  ADJUNCTS. 

(A  UNIVERSAL  SYSTEM  OF  ELECTRICAL  MEASUREMENT.) 
By  W.  CLARK  FISHER. 

The  extended  use  of  the  Potentiometer  System  of  Electrical  Measurement  will,  it  is  hoped,  be 
sufficient  excuse  for  the  publication  of  this  work,  which,  while  dealing  with  the  main  instrument, 
its  construction,  use  and  capabilities,  would  necessarily  be  incomplete  without  similar  treatment  of 
the  various  apparatus  which,  as  adjuncts,  extend  the  range  and  usefulness  of  the  whole  system. 

Electrical  testing  may  be  said  to  have  passed  through  two  stages.  First,  that  which  may  be 
called  the  elementary,  in  which  first  principles  were  evolved  ;  secondly,  the  adaptation  of  the  same 
to  the  needs  of  the  telegraph  and  cable  engineer.  But  with  the  advent  of  electric  lighting  and 
other  undertakings,  such  testing  might  be  said  to  have  passed  into  the  third  or  practical  and  com- 
mercial stage,  where  large  quantities  have  to  be  dealt  with,  and  where  the  old  order  of  things 
changeth. 

The  engineer  or  practical  man  demands  that  he  shall  be  shown  results  quickly,  plainly  and 
accurately  with  a  minimum  of  trouble,  understanding,  and  consequently  "  Time,"  and  on  that 
account  prefers — like  all  good  mechanics — to  have  one  good  instrument,  which,  once  understood 
and  easily  manipulated,  can  be  used  in  a  variety  of  ways  to  suit  his  needs.  It  is  to  this  fact  un- 
doubtedly that  the  "  Potentiometer  "  method  of  measurement  owes  its  popularity.  Its  accuracy 
is  rarely,  if  ever,  impunged.  Measurements  made  by  it  are  universally  accepted  amongst  engineers, 
and  it  might  be  well  termed  a  "  universal  "  instrument  in  "  universal "  use. 

Over  400  pages  and  200  specially  drawn  Illustrations.     Price  12s.  6d.,  post  free. 

SUBMAKINE  CABLE-LAYING  AND  REPAIRING. 

By  H.  D.  WILKINSON,  M.I.E.E.,  &c.,  &c. 

This  work  describes  the  procedure  on  board  ship  when  removing  a  fault  or  break  in  a  submerged  cable 
and  the  mechanical  gear  used  in  different  vessels  for  this  purpose ;  and  considers  the  best  and  most  recent 
practice  as  regards  the  electrical  tests  in  use  for  the  detection  and  localisation  of  faults,  and  the  various 
amenities  that  occur  to  the  beginner.  It  gives  a  detailed  technical  summary  of  modern  practice  in  Manu- 
facttBtoe,  Laying,  Testing  and  Repairing  a  Submarine  Telegraph  Cable.  The  testing  section  and  details  of 
boardsnfji,  fsaa&fip.  bave  been  prepared  with  the  object  and  hope  of  helping  men  in  the  cable  services  who  are 
looking  further  Into  these  branches  The  description  of  the  equipment  of  c*ble  ships  and  the  mechanical  and 
electrical  work  carried  on  during  the  laying  and  repairing  of  a  submarine  cable  will  also  prove  to  some  not 
directly  engaged  in  the  profession,  but  nevertheless  interested  fc  the  enterprise,  a  means  of  informing  them- 
selves as  to  the  work  which  has  to  be  done  from  the  moment  a  new  cable  is  projected  until  it  is  successfully 
laid  and  worked. 

The  Chapter  on  "  Testing  "  is  especially  valuable  and  up  to  date. 

1.  2  and  3,  Salisbury  Court,   Fleet  Street,  London,  E.C. 


The  Electrician"  Printing  and  Publishing  Co.,  Ltd.,      11 
"THE  ELECTRICIAN"  SERIES— continued. 


Over  300  pages,  106  illustrations.     Price  10s.  6d.,  post  free. 

The  ART  of  ELECTROLYTIC  SEPARATION  of  METALS 

(THEOEETICAL  AND  PEACTICAL). 
By    GEORGE     GORE,     LL.D.,    F.R.S. 

THE  ONLY  BOOK  ON  THIS  IMPORTANT  SUBJECT  IN  ANY  LANGUAGE. 


SYNOPSIS  OF  CONTENTS. 
HISTORICAL  SKETCH. 

Discovery  of  Voltaic  and  Magneto-Electricity — First  Application  of  Electrolysis  to  the 
Refining  of  Copper — List  of  Electrolytic  Henneries. 

THEORETICAL  DIVISION. 

Section  A. :  Chief  Electrical  Facts  and  Principles  of  the  Subject. — Electric  Polarity  and 
Induction,  Quantity,  Capacity,  Potential — Electromotive  Force— Electric  Current — Conduction 
and  Insulation — Electric  Conduction  Kesistance. 

Section  B. :  Chief  Thermal  Phenomena. — Heat  of  Conduction  Kesistance — Thermal  Units 
Symbols,  and  Formulae. 

Section  C. :  Chief  Chemical  Facts  and  Principles  of  the  Subject. — Explanation  of  Chemical 
Terms — Symbols  and  Atomic  Weights — Chemical  Formulae  and  Molecular  Weights — Belation  of 
Heat  to  Chemical  Action. 

Section  D.:  Chief  Facts  of  Chemico -Electric  or  Voltaic  Action. — Electrical  Theory  of 
Chemistry — Kelation  of  Chemical  Heat  to  Volta  Motive  Force — Volta-Electric  Eelationb  to 
Metals  in  Electrolytes — Voltaic  Batteries — Eelative  Amounts  of  Voltaic  Current  produced  by 
Different  Metals. 

Section  B. :  Chief  Facts  of  Electro-Chemical  Action. — Definition  of  Electrolysis — Arrange- 
ments for  Producing  Electrolysis — Modes  of  Preparing  Solutions — Nomenclature — Physical 
Structure  of  Electro-Deposited  Metals — Incidental  Phenomena  attending  Electrolysis — Decom- 
posability  of  Electrolytes— Electro-Chemical  Equivalents  of  Substances — Consumption  of  Electric 
Energy  in  Electrolysis. 

Section  F. :  The  Generation  of  Electric  Currents  by  Dynamo  Machines.— Definition  of  a 
Dynamo  and  of  a  Magnetic  Field — Electro-Magnetic  Induction — Lines  of  Magnetic  Force. 

PRACTICAL  DIVISION. 

Section  G.  Establishing  and  Working  an  Electrolytic  Copper  Refinery. — Planning  a  Kefinery 
— Kinds  of  Dynamos  Employed — Choice  and  Care  of  Dynamo — The  Depositing  Boom — The  Vats 
— The  Electrodes — The  Main  Conductors — Expenditure  of  Mechanical  Power  and  Electric 
Energy — Cost  of  Electrolytic  Kenning. 

Section  H. :  Other  Applications  of  Electrolysis  in  Separating  and  Refining  Metals. — Elec- 
trolytic Refining  of  Copper  by  other  Methods — Extraction  of  Copper  from  Minerals  and  Mineral 
Waters— Electrolytic  Refining  of  Silver  Bullion  and  of  Lead— Separation  of  Antimony,  of  Tin,  of 
Aluminium,  of  Zinc,  of  Magnesium,  of  Sodium  and  Potassium,  of  Gold— Electrolytic  Refining  of 
Nickel — Electric  Smelting. 

Appendix.— Useful  Tables  and  Data. 

Second  Edition,  price  %B.,  post  free. 

ELECTRO-CHEMISTRY. 

By  GEORGE  GORE,  LL.D.,  F.R.S. 

This  book  contains,  in  systematic  order,  the  chief  principles  and  facts  of  electro-chemistry 
and  is  intended  to  supply  to  the  student  of  electro-plating  and  electro-metallurgy  a  scientific  basis 
upon  which  to  build  the  additional  practical  knowledge  and  experience  of  his  trade.  A  scientific 
foundation,  such  as  is  here  given,  of  the  art  of  electro -metallurgy  ;s  indispensable  to  the  electro- 
depositor  who  wishes  to  excel  in  his  calling,  and  should  be  studied  previously  to  and  simul- 
taneously with  practical  working.  As  the  study  of  electro- chemistry  includes  a  kro-vleclge  not 
only  of  the  conditions  under  which  a  given  substance  is  electrolytically  separated,  but  also  of  the 
electrolytic  effect  of  a  current  on  individual  compounds,  both  are  described,  and  the  series  of 
substances  are  treated  in  systematic  order.  An  indispensable  book  to  Electro -Metallurgists. 

I,  2  and  3,  Salisbury  Court,  Fleet  Street,  London,  E.G. 


12      "The  Electrician  "  Printing  and  Publishing  Co.,  Ltd., 
"THE   ELECTRICIAN"   SERIES—  continued. 

Electrical  Laboratory  Notes  &  Forms. 

ARRANGED  AND  PREPARED  BY 

Dr.    J.    A.    FLE3JLIIVO,    M.A.,    FJR.S. 

Professor  of  Electrical  Engineering  in  University  College,  London. 

These  "  Laboratory  Notes  and  Forms  "  have  been  prepared  to  assist  Teachers,  Demonstrators 
and  Students  in  Electrical  Laboratories,  and  to  enable  the  Teacher  to  economise  time.  They 
consist  of  a  series  of  (about)  Twenty  Elementary  and  (about)  Twenty  Advanced  Exercises 
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Sheet  has  been  prepared,  two  pages  of  which  are  occupied  with  a  condensed  account  of  the  theory 
and  practical  instructions  for  performing  the  particular  Experiment,  the  other  two  pages  being 
ruled  up  in  lettered  columns,  to  be  filled  in  by  the  Student  with  the  observed  and  calculated 
quantities.  Where  simple  diagrams  will  assist  the  Student,  these  have  been  supplied.  These 
Exercises  are  for  the  most  part  based  on  the  methods  in  use  in  the  Electrical  Engineering 
Laboratories  of  University  College,  London  ;  but  they  are  perfectly  general,  and  can  be  put  into 
practice  in  any  Electrical  Laboratory. 

Each  Form  is  supplied  either  singly  at  3d.  nett,  or  at  3s.  6d.  per  dozen  nett  (assorted  or 
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NOW  READY.  —  Cheaper  edition  of  "Electrical  Laboratory  Notes  and  Forms."  These 
cheaper  Forms  have  been  prepared  for  the  use  of  students  and  teachers  at  the  Polytechnic  and 
other  science  classes  throughout  the  country.  These  new  Forms,  which  differ  only  from  the  higher- 
priced  set  in  being  printed  on  smaller  and  cheaper  paper  and  with  less  space  for  tabulated  records, 
are  issued  at  half  the  price  of  the  original  set. 


1.  The  Exploration  of  Magnetic  Fields. 

2.  The  Magnetic  Field  of  a  Circular  Current. 

3.  The  Standardisation  of  a  Tangent  Galvanometer  by  the  "Water  Voltameter 

4.  The  Measurement  of  Electrical  Resistance  by  the  Divided  Wire  Bridge. 

5.  The  Calibration  of  the  Ballistic  Galvanometer. 

6.  The  Determination  of  Magnetic  Field  Strength. 

7.  Experiments  with  Standard  Magnetic  Fields 

8.  The  Determination  of  the  Magnetic  Field  in  Air  Gap  of  an  Elecoro-magnet. 

9.  The  Determination  of  Resistance  with  the  Post  Office  Pattern  Wheatstone  Bridge. 

10.  The  Determination  of  Potential  Difference  by  the  Potentiometer. 

11.  The  Measurement  of  a  Current  by  the  Potentiometer. 

12.  A  Complete  Report  on  a  Primary  Battery. 

13.  The  Standardisation  of  a  Voltmeter  by  the  Potentiometer. 

14.  The  Photometric  Examination  of  an  Incandescent  Lamp. 

15.  The  Determination  of  the  Absorptive  Powers  of  Semi-Transparent  Screens 

16.  The  Determination  of  the  Reflective  Power  of  Various  Surfaces. 

17.  The  Determination  of  the  Electrical  Efficiency  of  an  Electromotor  by  the  Cradle  Method. 

18.  The  Determination  of  the  Efficiency  of  an  Electromotor  by  the  Brake  Method. 

19.  The  Efficiency  Test  of  a  Combined  Motor-Generator  Plant. 

20.  Efficiency  Test  of  a  Gas  Engine  and  Dynamo  Plant. 

ADVANCED     SERIES. 

21.  The  Determination  of  the  Electrical  Resistivity  of  a  Sample  of  Metallic  Wire. 

22.  The  Measurement  of  Low  Resistances  by  the  Potentiometer. 

23.  The  Measurement  of  Armature  Resistances. 

24.  The  Standardisation  of  an  Ammeter  by  Copper  Deposit. 

25.  The  Standardisation  of  a  Voltmeter  by  the  Potentiometer. 

26.  The  Standardisation  of  an  Ammeter  by  the  Potentiometer. 

27.  The  Determination  of  the  Magnetic  Permeability  of  a  Sample  of  Irom 

28.  The  Standardisation  of  a  High  Tension  Voltmeter. 

29.  The  Examination  of  an  Alternate-Current  Ammeter. 

30.  The  Delineation  of  Alternating  Current  Curves. 

31.  The  Efficiency  Test  of  a  Transformer. 

32.  The  Efficiency  Test  of  an  Alternator. 

33.  The  Photometric  Examination  of  an  Arc  Lamp. 

34.  The  Measurement  of  Insulation  and  High  Resistance 

35.  The  Complete  Efficiency  Test  of  a  Secondary  Battery. 

36.  The  Calibration  of  Electric  Meters. 

37.  The  Delineation  of  Hysteresis  Curves  of  Iron. 

38.  The  Examination  of  a  Sample  of  Iron  for  Magnetic  Hysteresis  Loss 

39.  The  Determination  of  the  Capacity  of  a  Concentric  Cable. 

40.  The  Hopkinson  Test  of  a  Pair  of  I'yn.nmos 


"The  Electrician"  Printing  and  Publishing  Co.,  Ltd.,      18 
"THE    ELECTRICIAN"    SERIES— continual. 


320  pages,  155  illustrations.    Price  6s.  6d.,  post  free. 

PRACTICAL  NOTES  FOR  ELECTRICAL  STUDENTS. 

LAWS,   UNITS,   AND   SIMPLE   MEASURING   INSTRUMENTS. 
By  A.  E.  KENNELLY  and  H.  D.  WILKINSON,  M.I.E.E. 

CONTENTS. 


Chapter  I.  —Introductory.  *V 

„        II.— Batteries. 

,,      III.— Electromotive  Force  and  Potential. 
„       IV.— Resistance. 
,,        V. — Current. 


Chapter  VI.— Current  Indicators. 
,,        VII.— Simple  Tests  with  Indicators. 
.,       VIII.— Calibration  of  Current  Indicators. 
,,          IX.— Magnetic   Fields    and   their   Measure- 
ment. 


NOW  Ready.—  Very  fully  Illustrated.     Price  10s.  6d. ;  post  free,  Us. 

Electrical  Testing  for  Telegraph  Engineers. 

By  J.  ELTON  YOUNG. 

This  book  embodies  up-to-date  theory  and  practice  in  all  that  concerns  everyday  work  of  the  Telegraph  Engineer. 

CONTENTS. 


Chapter  I.— Remarks  on  Testing  Apparatus. 
„        II.— Measurements  of  current,  Potential,  and 

Battery  Resistance. 
,,      III. — Natural  and  Fault  Current. 
,,       IV.— Measurement  of  Conductor  Resistance. 
,,         V.  -Measurement  of  Insulation  Resistance. 
,.       VI.— Corrections  for  Conduction  and  Insulation 

Tests. 


Chapter  VII.— Measurement  of  Inductive  Capacity. 
VIII.— Localisation  of  Disconnections. 
IX.— Localisation  of  Earth  and  Contacts. 
X.  —  Corrections  of  Localisation  Tests. 
XI.— Submarine  Cable  Testing  during  Manu- 
facture, Laying  and  Working. 
XII.— Submarine  Cable  Testing  during  Localis- 
ation and  Repairs. 


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DRUM  ARMATURES  AND  COMMUTATORS 

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The  New  Edition  forms  a  complete  Illustrated  Treatise  on  Hertzian  Wave  Work.  The  Full  Notes  of  the 
interesting  Lecture  deuveied  by  the  Author  before  the  Rnya>  Institution  in  June,  1894,  form  the  first  chapter  of 
the  book.  The  second  chapter  is  devoted  to  the  Application  of  Hertz  Waves  and  Coherer  Signalling  to 
Telegraphy,  while  Chapter  3  gives  Details  of  other  Telegraphic  Developments.  In  Chapter  4  a  history  of  the 
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or  X-Rays. 

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CARBON  MAKING  FOR  AIL  ELECTRICAL  PURPOSES, 

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Lighting,  Electrolytic,  and  all  other  electrical  purposes. 

CONTENTS. 
Chapter       I.— Physical  Properties  of  Carbon.  I  Chapter  IX.— The  Estimation  of  High  Temperatures. 


II.— Historical  Notes. 
III. — Facts  concerning  Carbon. 
IV.— The  Modern  Process  of  Manufacturing 

Carbons. 
V.— Hints  to  Carbon   Manufacturers   and 

Electric  Light  Engineers. 
VI.-A  "  New"  Raw  Material. 
VII.— Gas  Generators, 
VIII.-The  Furnace. 


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Carbon    Works    and   the    Profits    in 
Carbon  Manufacturing. 
XII.— The  Manufacture  of  Electrodes  on  a 

Small  Scale. 

XIII.— Building  a  Carbon  Factory. 
XIV.— Soot  or  Lamp  Black. 
XV. -Soot  Factories 


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graphic purposes,  which  were  at  that  time  the  principal  electrical  industries,  to  resuscitate  "  THE  ELECTRICIAN  " 
and  once  again  establish  it  as  the  recognised  leading  organ  of  electrical  science  and  industry.  It  will  thus  be 
seen  that  "  THE  ELECTRICIAN  "  is  the  oldest  electrical  journal  published.  Since  its  second  advent  "  THE  ELFC- 
TRICIAN"  has  made  rapid  progress,  and  has  continued  to  record  in  a  full  and  thorough  manner  all  the  iireat 
discoveries  and  experiments  in  electrical  science,  and  to  illustrate  how  these  could  be  commercially  applied 
and  profitably  worked.  In  the  volumes  of  "  THE  ELECTRICIAN  "  wiil  be  found  a  valuable  collection  of  ori-inal 
papers  and  articles  by  nearly  every  leading  writer  on  electrical  matters,  including  :— 
Prof.  W.  G.  Adams,  Prof.  J.  A.  Ewing,  Prof.  P.  Lenard,  i  Prof.  H  J  Rvan 

™.  „    T    A.^K.,^          ^  „    A    ^.._.  ,,_  TT__.  ,    a.r  Dav:dJgS5oninonS) 


Mr.  G.  L.  Addenbrooke, 
Sir  James  Anderson, 
Mr  Hollo  Appleyard, 
Prof.  H.  E.  Armstrong, 
Signer  R.  Arno, 
Mr.  L.  B.  Atkinson, 
Prof.  W.  E.  Ayrton, 
Mrs.  Ayrton, 
Mr.  J.  Mark  Barr. 
Prof.  W.  F.  Barrett, 
•Mr.  W.  W.  Beaumont, 
Mr.  A.  R.  Bennett, 
Mr.  Shelford  Bidwell, 
Mr.  T.  H.  Blakesley, 
Herr  0.  T.  Blatby, 
M.  A.  Blondel, 
Mr.  Bertram  Blount, 
Prof.  J.  C.  Bose, 
Mr.  C.  Vernon  Boys, 
Mr.  G.  J.  Burch, 
Mr.  A.  Campbell, 
Major  P.  Cardew, 
Prof.  H.  S.  Curhart, 
Mr.  E.  T.  Carter, 
Mr.  G.  M.  Clark 
Mr.  W.  R.  Cooper, 
Mr.  R.  E.  Crompton, 
Sir  W.  Crookcs, 
Mr.  A.  Dearlovo, 
Mr.  P.  B.  Deluuy 
Prof.  J.  Dewnt, 
Herr     M.     Dolivo     Dob- 

rowolsky, 

Herr  Alfred  Dubsky, 
Dr.  Louis  Duncan, 
Mr.  J.  Elster, 
Mr.  W.  B.  Esson, 
Mr.  Sydney  Evershed. 


Mr.  C.  A.  Faure, 
Prof.  G.  Ferraris, 
Mr.  W.  C.  Fisher, 
Prof.  G.  F.  FitzGerald, 
Dr.  J.  A.  Fleming, 
Prof.  George  Forbes, 
Prof.  J.  Frith, 
Prof.  W.  Garnett, 
Mr.  J.  Gavey, 
Mr.  W.  Geipel, 
Prof  H.  Geitel, 
Mr.  A.  H.  Gibbings, 
Sign  or  G.  Giorgi, 
Dr.  J.  H.  Gladstone, 
Mr.  R.  T.  Glazebrook, 
Dr.  George  Gore, 
Mr.  A.  Hay, 
Mr.  Oliver  Heaviside, 
Herr  J.  S.  Hechr,, 
Prof,  von  Helmholtz, 
Dr.  H.  Hertz, 
Herr  A.  Hey  land, 
Prof.  W.  M.  Hicks, 
Mr.  Paget  Higgs, 
Dr.  Edward  Hopkinson, 
Dr.  John  Hopkinson, 
Prof.  E.  J.  Houston, 
Prof.  D.  E.  Hughes, 
Mr.  C.  C.  Hawkins, 
Mr.  Hugh  Erat  Harrison, 
Prof.  Dugald  C.  Jackson, 
Dr.  W.  Jaeger, 
Dr  G.  Jaumann, 
Mr.  Gisbert  Kapp, 
Prof.  A.  B.  W.  Kennedy, 
Dr.  A.  E.  Kennelly, 
Mr.  J.  B.  C.  Keishaw, 


Dr.  J.  Larmor, 
Dr.  S.  Lindeck, 
Dr.  Oliver  Lodge 
Mr.  L.  B.  Marks, 
Prof.  J.  Clerk  Maxwell, 
Mr.  P.  V.  McMahon, 
Prof.  Mengarini, 
Prof.  G.  M.  Minchin, 
M.  Henri  Moissan, 
Mr.  W.  M.  Mordey, 
Dr.  Alex.  Muirhead, 
Prof.  F.  E.  Nipher, 
Mr.  H.  F.  Parshall, 
Prof.  John  Perry, 
Mr.  Nelson  W.  Perry, 
Mr.  C.  E.  S.  Phillips, 
Mr.  W.  H.  Preece, 
Dr.  C.  S.  du  Riche  Preller, 
Mr.  W.  H.  Pretty, 
Mr.  W.  A.  Price, 
Prof.  J.  H.  Poynting, 
Dr.  M.  I.  Pupin, 
Mr  G.  S.  Ram, 
Prof.  W.  Ramsay, 
Mr.  F.  C.  Raphael, 
Mr.  H.  W.  Ravenshaw, 
Mr.  J.  S.  Raworth, 


Mr.  W.  B.  Sayers, 

Dr.  Paul  Schoop, 

Mr.  Louis  Schwendler, 

Mr.  G.  F.  C.  Searle, 

Mr.  J.  S.  Sellon, 

Mr.  Alex.  Siemens, 

Dr.  Werner  von  Siemens 

Mr.  M.  Holroyd  Smith, 

Mr.  Willoughby  Smith, 

Mr.  Albion  T.  Snell, 

Mr.  W.  H.  Snell, 

Dr.  W.  Spottiswoode, 

Mr.  J.  T.  Sprague, 

Prof.  Balfour  Stewart, 

Mr.  A.  Still, 

Prof.  W.  Stroud, 

Dr.  W.  E.  Sumpner, 

Mr.  James  Swinburne, 

M  r.  A.  A.  C.  Swinton 

Mr.  F.  A.  TaUor, 

Mr.  Nikola  Tesla, 

Prof.  Silvanus  Thompson, 

Prof.  Elihu  Thomson, 

Prof.  J.  J.  Thomson 

Sir  Wm.  Thomson  (Lord 

Kelvin), 
Mr.  H.  Tomlinson, 

Mr.  J.  Rennie, 

Mr.  W.  G.  Rhodes, 

Prof.  A.  Righi, 

Mr.  G.  H.  Robertson, 

Prof.  W:  C.  Rontgen, 

Mr.  Gaston  Roux, 

Prof.  Rowland, 

Prof.  Riicker, 


Lord  Rayleigh, 
nie, 


.  , 

Mr.  Warren  de  la  Rue. 
Mr.  Alex.  Russell 


Mr.  A.  P.  Trotter, 
Prof.  John  Tyndall, 
Mr.  E.  J.  Wade, 
Mr.  F.  C.Webb, 
Mr.  F.  M.  Weymouth, 
Mr.  H.  D.  Wilkinson, 
Mr.  E.  Wilson, 
Mr.  J.  Elton  Young, 
Dr.  Zetzsche, 

&c.,  &c.,  &c. 


Mr.  Hamilton  Kilgour, 

And  all  papers  read  before  the  principal  Electrical  Institutions  throughout  the  world  by  men  eminent  in  the 
Electrical  Profession  have  been  given,  together  with  authenticated  renorts  of  the  discussions  thereupon. 

In  addition  to  the  above,  "  THE  ELECTRICIAN"  forms  a  complete  record  of  all  the  important  legal  investi- 
gations which  have  occupied  the  attention  of  the  Courts  of  Justice  for  the  past  14  years,  and  it  is  customary  for 
"THE  ELECTRICIAN  "  to  occupy  a  prominent  position  in  the  Courts  as  an  authority  upon  all  questions  affecting 
the  Electrical  Profession.  In  this  connection  it  is  only  necessary  to  point  to  the  actions  which  have  arisen  from 
time  to  time  upon  the  Edison  and  Swan  Patents,  the  Compound  Winding  Patents,  the  High  and  Low  Tension 
Systems,  the  Accumulator  Patents,  the  Telephone  Patents,  etc.,  etc.,  in  which  "  THE  ELECTRICIAN"  has  figured 
as  a  reliable  authority,  and  has  been  put  in  evidence  and  accepted  by  the  parties  concerned. 

A  regular  feature  in  "  THE  ELECTRICIAN  "  has  always  been  the  verbatim  reports  of  meetings  of  Electrical 
Companies  and  Corporations,  and  while  "  THE  ELECTRICIAN  "  has  never  trenched  upon  the  grounds  legitimately 
occupied  by  financial  journals,  the  information  of  a  financial  character  given  from  week  to  week  in  its  columns  is 
full,  reliable,  and  absolutely  unbiassed.  It  has  no  interest  whatever  in  any  financial  schemes,  and  is  devoted 
entirely  to  the  interests  of  the  profession  it  was  established  to  serve.  "THE  ELECTRICIAN"  gives  full  reports 
of  Meetings,  &c.,  held  on  Thursdays,  so  that  subscribers  interested  obtain  their  copies  by  Friday  morning's  post. 

The  original  articles  appearing  in  "  THE  ELECTRICIAN  "  are  written  by  gentlemen  having  no  interest  what- 
ever in  particular  electrical  systems,  and  with  but  one  object,  and  that  the  advancement  of  electrical  knowledge 
and  electro-technology  generally.  Many  of  these  original  series  of  articles  have  since  been  revised  and  amplified 
by  their  authors,  and  published  in  book  form.  These  form  the  nucleus  of  the  well-known  "Electrician" 
Series,  of  which  further  particulars  will  be  found  herewith. 

Finally,  "  THE  ELECTRICIAN  "  has  been  of  incalculable  service  to  technical  education,  and  has  done  much 
to  make  the  general  study  of  electricity  the  reality  it  has  undoubtedly  become.  No  aspirant  to  honour  and 
renown  in  the  electrical  profession  can  hope  to  keep  abreast  of  the  never-ceasing  stream  of  discoveries  of  new 
applications  of  electrical  science  to  every  day  commercial  pursuits  who  does  not  diligently  peruse  the  columns  of 
"  THE  ELECTRICIAN,"  which  is  pre-eminently  the  leading  electrical  journal. 


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HALF-YEAR. 

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O16    O 


QUARTER. 

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O    8    O 


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